I can only help with problem 2. You need to get yourself added as an additional signatory on his accounts. This may require physically going to a bank branch with him. Then you should be able to create separate user accounts without logging in to your father's profile.
For the other problems you're probably going to have to establish a conservatorship. Consult an attorney in your father's state who has experience in elder law.
My mother died of dementia.
Let me tell you, it doesn’t get easier and it doesn’t get better. It only gets worse every day. Be prepared for an absolute shitshow. Dementia isn’t just forgetting stuff. It’s progressive brain damage that leads to things like them not being able to care for themselves, becoming incontinent and being unable to swallow. The financial situation is the least of your worries but also the greatest of your worries because there are no facilities that are affordable that let your dad live his last years of his life with dignity. If you haven’t already I suggest reading up on it and the effects that dementia had on their caretakers.
Are you the only one available to take care of him? Does he not have family around him to take care of him? He will not be able to survive on his own in a few years, if not already.
You need to get power of attorney over his finances. That means you control everything. Depending on where he is at, and what you can afford, he will need to be in medical care in a few years. It can either be in a memory facility with full care which is very expensive, or you can try taking care of him yourself but you will still need to hire a nurse to bathe him as he becomes incontinent, or brush his teeth, etc. If where you live has universal health care like Canada, you’ll find that it’s really not helpful and your own cheaper options are basically mental institutions that are horrible. But the waitlist is years long so you need to be prepared for this.
My absolute condolences to you, your dad and your family. Dementia is the cruelest diseases. If I find myself going down this path I will definitely commit suicide to relieve the burden from my family. In Canada they won’t let you end your life with medical assistance if you have dementia which is governmental stupidity to the extreme. If anything people with dementia need that option more than anyone because it will never get better.