What legal documents do I need for mom who lives with me and has Alzheimer's?

Call your local SS office. There is a procedure for “representative payee”.
Here is a link to explain.
https://www.ssa.gov/payee/

However, if your mom’s check is being automatically deposited into her bank account you shouldn’t have any problems with social security. Well...in these times who knows what trouble can show up.

At the bank your mom can okay you being POD (payable upon death) and you can also be added to her account if she chooses so you can pay her bills by check or debit card. Do not commingle your funds.
Some banks have their own POA document. Check with her bank to see what they require for you to handle her business. Get it all figured out in advance and then take her by for a set appointment for any signatures required.

Be careful if using prefabricated POAs that one prints out online and then signs in front of a notary.

You need to read them carefully. Look at several to see similarities and differences. Google what should a POA include and topics of that nature. On this site you can go to the search and find articles on POA as well.

Many lawyers offer packages for as little as $250/$500 that will include a will, both POAs, Living will. All the forms needed for simple estates such as your mom’s appears to be. But states differ so check locally. Also hospitals generally have DNR forms that are free for the asking.
My aunts attorney will even update hers if needed. One of the alternate POAs passed away and she added a different person afterwards. No charge. Most attorneys use templates as well. The advantage is they use documents they have experience defending or interpreting in their local area.
You can call around to different attorneys and price shop to get the going rate if that will help you decide.

There is a degree of comfort in having an attorney that you have a relationship with and can call when needed.

Note: You check private messages by clicking on your avatar and then clicking on profile.

This is not going to be a complete list. You say you have advanced directives? Did she designate you as POA for both health and financial matters. Once she is formally diagnosed with dementia, she can no longer make you POA. Are you spending her money for her? You need to keep records as to how that money is spent. If you take a portion for rent, which you could do, you should account for it. You did not give details. You say she has Alzheimer’s. Has she been tested? Has she been declared not competent to make decisions? If so, and if you are not already her POA, then she may need a guardian to make decisions for her. I think it would be best to have a meeting with an elder care attorney who can advise you of what you need based on your situation and state. This could be done over the phone or video conference as I would not want to wait.

Just a heads up - even if you have an established POA over financial matters it will not be sufficient for social security and some banks, which have their own forms.

Power of Attorney (for financial and legal matters) and Health Care Proxy (to make her healthcare decisions and gain access to her medical records). These documents are priceless. Try to get them ASAP. A lawyer cannot legally execute these documents if they don't feel the person is competent to make their own decisions. We did these when my mom had her Alzheimers diagnosis but was still able to legally consent.

OneBlueMoon, check your Private Messages -- I sent you one.

OneBlueMoon
in regard to your mom wanting to handle her own money. (I’m assuming she has a bank account maybe not) That’s what the banks and SS office and insurance want as well. When they have to deal with someone else their risks go up.
Your mom is handicapped in multiple ways. In order for her to continue to use you as her way to get by in life then she has to give you official, legal permission that is recognized by the bank and SS office and insurance. The individuals you deal with at those locations don’t make the rules and neither does your mom..,, and neither do you. So understanding the institution requirements is a must.
Your mom either trusts you to do what’s right for her or she doesn’t. The longer you wait for her to understand that, the less ability she has. Time is not her friend.
Use whatever works to paint the picture FOR HER to understand that she has to give you the recognized authority to interface with the world for her as she loses the ability to do so for herself.
A couple of ideas for you on SS. Write a proper letter (including date, SS#, BD and full name for each of you, explaining her problem and have her sign it, along with you and drop it in the box at the SS office. You can walk in and drop this off at the SS office I use. I don’t know about yours. You don’t wait for an agent, just drop it in the box inside usually located within sight of the front desk. In the letter ask for a telephone appointment and explain that if possible you would like someone who can speak her language to call. Leave your phone number, best time to csll, and keep a copy of the letter.
You May want to become representative payee for the reasons discussed (especially if she doesn’t have a bank account). It’s more paperwork on you but perhaps less stress overall.
For her insurance just call them with mom handy. Tell them mom wants you to speak for her and would they please send any paperwork mom needs to sign granting them permission to allow you to speak for her. Sometimes offices will take a verbal approval over the phone but that is usually only for that particular call. Of course you need to have identifying information handy to make it easy for them to know they are truly dealing with your mom and her representative.
I’m not defending the SS rep but they do have a stressful job. I had to go three times to get a problem taken care of for my husband so I do understand that they try to keep you with the same rep but the reps do have managers you could ask for.
I don’t use the SS office nearest me. It’s too busy. But I imagine they are all overwhelmed at this point.
Ask the rep what you could do to make it easier for all of you to get your business taken care of when next you speak.

If you haven't already done so, I would suggest contacting an elder law attorney. Typical documents needed for an elder are durable power of attorney, advanced directives, and a last will and testament. Given the fact she has Alzheimer's disease, she most likely unable to execute these documents due to her inability to comprehend their meaning. I'm thinking you may need guardianship. However, if you are concerned about managing matters upon her death, if you are the only or oldest child, you may not have any difficulty managing her affairs. State law may also factor in this. If you can't afford an elder law attorney, legal aid or the area agency on Aging may be helpful too.

Reporting/receipts:

"If I become representative payee, how do I document her spending? Portion of her income goes towards the rent and the rest is for her grocery and other needs. I usually purchase from the bulk store for everyone in my family. How do I document her portion? Do I show them the whole receipt? How to document her rent? What is the least complicated way to deal with documenting her expenses since I know the SS office will ask that?
Also, is being the representative payee, similar to POA for SS office so I don't have to drag her there everytime or talk on the phone with them on her behave?"

(One thing that would be good to have in hand when you see atty regarding POA is a simple care-giving contract (rent, etc.) This is more a CYA for you.)

The SS mails a form at the end of the year. Although you can fill it in and mail it, they also allow you to file it online. DO keep records of ANYTHING you spend her money on, but you DO NOT have to send any of these records. They are just for backup in case SS questions anything. If at all possible, try to split the purchases so you have separate receipts OR calculate, and keep records, on what % of these purchases would benefit her.

This is difficult, since she lives with you, so meals, laundry, etc are probably co-mingled, but if you see an attorney for the POA, they can probably guide you on what would be a reasonable %age could be allocated.

Again, the questionnaire is just the "big" picture, such as how much for housing, food, medical, incidentals, etc. It really is simple, so long as you keep track of her funds.

I also finished reading the other posts, and I do see those who say they didn't do the rep payee because it is a hassle. Although SS doesn't approve and can put that scope up your butt if they so choose, it *can* be done... however, if you move, have to change anything, etc., you have NO authority.

Sure, some say go online and create account on SS page for LO - DO understand this is considered FRAUD. Most online account applications for ANYTHING mandate that you VERIFY YOU are that person, and you are NOT. It is even MORE crucial when this is a federal system. The biggest hassle I had was getting the account open, because the CU doesn't do this often (likely because people circumvent around it, illegally I might add) and had difficulty doing it, even with their own "cheat sheet", aka How-To...
I don't have mom's in front of me, but it was only ONE page, less than a dozen questions, some just confirming you, mom, etc. They ask how much of her SS is spent on several categories, such as housing, food, incidentals, medical, etc. They also ask how much, if any, is saved in an account. This questionnaire is VERY simple to process - just keep receipts and any care-giving contract in a safe place. More than likely you will never need them.

This past year, SS sent a followup questionnaire to me, because I listed ALL of her income going to housing/meals. She lives in a facility and the cost is more than double her pension and SS income, so to keep it simple I list both as going towards that and any funds from her trust cover the balance and incidentals. I laughed at the statements in the letter!!! Oh, most people have other needs, going out, buying things, vacations, etc... Seriously - the pittance she gets probably would not even rent her a regular apartment, never mind food!! On top of that, she is 96, has dementia, hearing loss, macular degeneration and most recently refuses to stand or walk without major assistance!!! Where do they think she's going, to Australia for a well-earned vaca????

I also lambasted them because based on her income for last year (her share of the condo sale drove her income over the limit) they jacked up her Medicare. Tax consultant told me it will go back down next year, after this year's filing. Still.... bleed our responsible elders (HER savings are keeping her off Medicaid!!!) to death, but ...

OneBlueMoon,
Durable power of attorney is essential.
Some how you need to have one drawn up. Include medical and financial. It needs to be signed by her and notarized.
After that, you should be able to carry out her affairs. Many times you will have to get a copy of DPOA to do her business. It can be daunting, but doable.
Although I am still new at this, I was able to file my Aunts taxes, set up auto pay for her bills, sell her house etc...
If Mom is unable to sign anything, you may have to seek legal guardianship.
Hope this helps!
Best of luck!
God bless!!!

Message below From OneBlueMoon
she needs help from someone sho has actually done the Rep Payee. I have not.

“If I become representative payee, how do I document her spending? Portion of her income goes towards the rent and the rest is for her grocery and other needs. I usually purchase from the bulk store for everyone in my family. How do I document her portion? Do I show them the whole receipt? How to document her rent? What is the least complicated way to deal with documenting her expenses since I know the SS office will ask that?
Also, is being the representative payee, similar to POA for SS office so I don't have to drag her there everytime or talk on the phone with them on her behave? 
Thank you.”