Hello, I am a solicitor with 20 years experience. I will try to help you with this.
I thin she can prevent anyone she wants coming inter what I assume is her home as well as your father's. You will, I am afraid, just have to play along with it and arrange to see your father in whatever convenient locations you can arrange.
got it. so presumably, even though the house belongs to my father, as his wife, she has the same rights of access denial as he does, even though he is happy to see me in his home...
Pretty much yes. If there is ever a situation where your father is saying you can come in and she is saying you cannot then it may be that you can accept your father's invitation as long as no offences will take place (eg assaults, breaches of the peace etc.). But it sounds as though (probably due to your father's ill health) she is controlling access to him and to the house.
As far as the financial abuse is concerned it may well be worth contacting Age Concern about this who I would think would be able to give useful general advice.