I'm POA and expenses like Uber and house repairs have gotten mixed between my account and Pop's. How do I amend and repay/balance?

Yes, I have heard of the 'All Debts Paid' document. You will not need to do this right away though. Save all receipts.
I'm sure you also know that as POA you are entitled to being compensated for every hour you put in on your grandfather's behalf and for his benefit.
The probate court in the town your grandfather lives in can tell you how much you are allowed to charge per hour for the POA duties. You must also keep a record of these hours spent.
You do not owe him any money, nor will you owe any if he has to be applied for Medicare. Did you make a public declaration before witnesses when you began working as his POA? You probably didn't, so therefore you've been working as his POA for some time. Understand? ALL the receipts count.
The environment you're working as POA in sounds ridiculous. Like something right off the Jerry Springer Show. When I was POA for my father it was the same thing.
I didn't do it for free and neither should you. Make a record even if you have to estimate of how many hours per month you spend working for your grandfather on his behalf as his POA.
This means time spent on the phone, errands, doctor's appointments, paying bills, doing banking, and/or communicating with anyone on his behalf. After the probate court tells you the amount you're entitled to charge, start withdrawing it from his bank accounts.
My father had to go into a nursing home. He also had to go on Medicaid. The nursing home didn't like it, but I paid myself what I was owed while there was still money to pay me with. I never allowed the NH to have access to any of his bank accounts. I knew that they were expecting all of his monthly income in cash. If I had let them take it, I would not have been paid my monthly POA fees. All of it was perfectly legal too.
Please visit the probate court and talk to them. You won't be sorry. Ask your lawyer too.

https://www.fiscaltiger.com/paid-in-full-letter-and-template/

I'm not familiar with the form, but that lawyer should know where to direct you. Also, you may want to try city hall. If you are living in the same roof, I wouldn't worry so much about comingling accounts and sharing costs/expenses. I went through it, with the same worry as you, but ultimately didn't need any naturalized form or anything. They understood it as normal household expenses. If the debts are larger though, in the mid/high 4 figures and up, you may want to try to rectify. Even still, a good Medicaid caseworker can push it through, or if for some reason Medicaid is "denied", and grandfather is in a position where he needs 24 hr care in a NH, and can't get it at home, you either place him int the hospital or call APS. APS will take him out of a dangerous situation and ensure his safety. The downside to that is family will likely lose all control over where he goes and if you can visit him(if they accuse of abuse/neglect)