Has anyone successfully contested POA?

My stepmother has POA for my Dad, diagnosed with sudden onset Vascular Dementia early this year. This wouldn't be an issue for me except that for some reason she has a phobia about having him come home. Dad is only 70 and up until his hospitalisation was still successfully running his own business. Overnight his life has been turned upside down and he now lives in a room in a Care Facility where all the other residents are in a far more advanced state of "old age". His wife does visit him almost every day (along with a million of his friends and family) and I DO believe she loves him but I DON'T believe he should be living apart from her and the only reason she seems to be able to offer on the subject is that she is scared he will get hurt while she is looking after him. She also has an acute phobia surrounding her belief that he can't shower himself (something that I think came up in his four weeks in hospital and which I think was then limited to having to turn the taps on and hand him the soap - I don't even know if this is still the case). (Showers are definitely a phobia - my father-in-law is home from a stroke and my stepmother couldn't believe my MIL was looking after him by herself "He must be able to shower himself," she said.) She loses it any time anyone suggests that he should be at home but won't elaborate on why he needs to be locked up. He is still highly intelligent and able to have totally with it conversations with us on just about any subject. His personality and his ability to converse hasn't changed since before the onset. He knows he has vascular dementia and that it's affecting his memory. But he still makes new memories - they sometimes take a bit longer to come to the fore but they do pop up every now and then. And he's bored. He's not a reader, he's not a sit and visit for hours on end person - he's a skilled worker and he can't do anything of interest in the care facility due to not being able to have the tools there. (And I have tried to organise attendance at a specialised workshop but aside from the possibility that he might not be as advanced in his dementia as some of the attendees my stepmother does not think he needs to go). My stepmother has gotten to the stage where she's happy to take him home with her during the day but she wants to "return" him to the facility at night (which is what she had decided would happen when he was in hospital but at the time he was so panicky he was assessed as needing secure care - which has since changed). It's good for him that she's gotten to this stage but she has no intention of having him home full time. I don't think it's fair that "she" is spending "his" savings and proceeds from his recently sold business and house on keeping him in care (they have separate funds) when he would be so much happier at home - they could spend that money on having carers in to shower him and keep him company when she goes out - that would be a better spend. Dad thinks the same. We have been speaking about it quite frequently on his visits to me lately (I take him out for a day a week - I am not allowed to have him to stay over as he "will panic if he wakes in an unfamiliar place" - her words). He is able to talk about it quite rationally - he talks about everything rationally - he just cannot always remember that he spoke about it the next day unless you offer him a prompt - he also KNOWS that he will probably forget about it. I know others with Dementia who were living at home and some even by themselves WAY past the point that my Dad is at. Dad actually specifically said that even if she needed to go out and leave him at home he would know not to touch anything electrical or make himself a cup of coffee or anything - he's desperate to go home. Another option is for him to come and live with us. There would have to be major changes which my family is happy to make however we have yet to ask Dad if this would be something he would be happy with (he WAS happy with coming to stay over the weekend until my stepmother stomped on that) and I KNOW he would be happier with my stepmother. To do this however I would have to have POA, at least over his health (I don't care if she has the POA over his money as long as it only gets used on him). And that's because there is no way she will let him come and live with us. I think she's extremely selfish. She is liking the attention that comes with being the person in charge of this man that everyone adores. She tells his friends and family what to do and when they can see him, she has most of the contact with the facility although they are starting to talk to me a bit now and all of the contact with doctors . She has always been controlling and when she makes a decision it's a hard thing to change. But I've been told that getting a POA changed like this is very, very unlikely. I'd like to hear from anyone who has tried, whether they've been successful or unsuccessful. I just want my Dad to be happy.