Help elderly parent pay mortgage or buy their home? Complicated

I am considering ways I can help my elderly dad - should I help him pay his mortgage or should I consider buying the house or ?

It's a little complicated. My wayward sibling pushed my parent into a reverse mortgage a few years ago. I just found out that this sibling did it a second time and increased the amount to 700k. But with the high rate of accumulated interest, my dad now owes almost 900k on his home worth just 1.3m to 1.5m (just 4 years ago he owed only 250k, which gives you some idea of what sibling has taken). And it's going up by almost 50k a year. Yes we are in contact with an attorney and maybe we can get his money back (any experience with elder financial abuse cases also welcome - for example lawyer vs using Adult Protective Services, civil vs criminal etc).

But now my dad (and I) are freaking out at how quickly the reverse mortgage is accumulating. We are considering refinancing into a conventional loan which would require us to pay off the 900k reverse mortgage he owes and get a conventional loan for 900k. But is this a good idea? He's in his late 80s and was very healthy but now declining (thanks to the stress). That way at least his interest would be halved, and we would feel more in control of his home.

From just casual searches, it looks like a 30 year loan for 900k would be about $4,300 per month but his total income is about 5.5k per month (although we could increase his distribution from his IRA). I could also contribute 1k per month but only as a loan, I can't afford to give the money away. Or is an adjustable better - and if so should we do a 3/1 or 5/1 or 7/1? The mortgage payment then goes down to 3.5k per month.

I also wondered if I should buy his home and he could pay me rent - or is that not a good idea? I wouldn't mind owning the home after he passes on but I have heard there are tax complications with that. Or should we just sell it while he still has some equity left. If we wait a few years, plus agent fees, he'll have no equity left and will get nothing.

Any thoughts from past experiences would be much welcome! Or great mortgage brokers who could help us understand. Or experience with Elder Financial Abuse.