I am going to assume you are in the USA. Yes, you are right because this disease only gets more difficult as time goes on. Mom is going to need more and more help. You have no choice but to accept what help others give and try not to resent what they don't do. The feelings of anger and resentment do not damage anybody but you. You do need to ask for help rather than expect people to offer help. If you have one, like your brother, who just do not participate, then you have to accept that. Holding on to negativity about what he will not do only depletes your energy, which you need to help your Mom. The sooner you can let it go the better.
I am sorry Mom broke her foot. The doctor is absolutely right. Many elderly dementia patients who are put under anesthesia, do have a decrease in their cognition. It is best not to do surgery if at all possible. I am sure it is more difficult for Mom with both feet in boots. If it is a minor break, it will probably heal well. I had much the same situation with Mom's right hand. It healed quickly, without surgery, and with no after effects.
Medical and Financial POA should be done while Mom can still sign and understand what she is signing. If she can no longer do this you will need to get guardianship through the courts. At least that is the way it is in NC. You do need to see a good Elder Lawyer ASAP... like last year! Here Heath Care POA is actually the most difficult because of the implications. When your Mom become incapacitated you have total responsibility for her health decision... and that is a huge responsibility. Each state is different so please check with an Elder Lawyer.
Each state has it's own laws related to Medicare and what they will pay for. It does have to be prescribed by her doctor and he will be able to tell you if it is allowed. Then it comes from one of the approved medical supply companies. Your best bet is to check with the doctor, have him prescribe, and ask him to help you get it.
Again, be thankful for the help you do have. Remember to ask for what you think they can do and don't just wait for them to see what you see.
Hope Mom's foot heals well... and hopefully something I have said is helpful