On the banking stuff it is much easier to keep everything separate. Initially my mother was far from happy, but she had been giving bank details out to random people who called, so I had little option.
Speak to the manager at the branch he goes into, who presumably is aware of problems. (If no branch simply go ahead with the rest, bank call centres know nothing about POAs.) GetAttorney access to his account. Ie your own check book and card and access to internet banking. Cancel hers. Decide whether to set up a parallel small savings account, one that allows a debit card but not overdraft, or simply give him cash every week. Banks are caught in a difficult bind. They need to protect vulnerable people from fraud, but obviously until capacity is proven not to exist (which is complex) they presumably have the capacity to make bad decisions. Then remove anything with bank details from her flat. Hopefully she wont remember them or passwords, so wont make much sense should she call. Plus the attorney debit card is in your name.
My mum ranted for about six months, but then forgot. I used to tell her it was the bank's decision. (Which it was slightly true as I had had to unravel all the dodgy direct debits.) I wrote down a summary of her assets to reassure her that she was not broke - poverty in old age was clearly that worried her deeply, most probably from her childhood.
I pay as much as I can by standing order or bank transfer, or use the debit card. If I pay cash for something I normally transfer the amount immediately using my phone app. Once every six months or so I calculate approximate mileage (my mother is two and a half hours away) and credit myself for that. I don't charge for my time though the wording of the POA allows for it. In short everything goes through her account and I keep her and my finances separate. This saves a lot of paperwork.
Attorney rules state that Attorneys are not allowed to borrow or lend, which means banks are unlikely to allow overdrafts or credit cards. This can lead to liquidity problems, as the only way to get a decent return on savings is via longer term deposits, yet expenses (I have just had to organise some emergency 24 hour care, as my mother is a a bit poorly and very confused, in the same month as a huge tax bill, but equally boilers can need replacing etc) come up randomly. In those circumstances I tend to transfer a round sum into her account and then transfer back the same round sum as soon as I can, rather than meet bills.
My mother is very well off. And my sibling is too busy earning a high salary to help. I feel strongly that whilst I need to support my mother, my family do not need to subsidise her and my brother. If she were less well off, I think I would try to broker a deal with my brother so he contributed to general expenses. It seems wrong that the one who does most, ends up out of pocket.