if it goes missing I will be held responsible (and probably have to pay him back)
Really? If I really thought this, would tend to take a step back and say that I did not know what to do for best. A mistake has been made, its not ideal but the burden should really not fall on you.
I have no idea why I'm so involved in this!!!
BIWI had some clues for you in her advice to stop enabling him, which means take a step back, leave him to it. I get that it is not easy, but it is possible. Are you dependent on them, in which case get yourself a job and some independence.
Ive read more of your thread now, OP, and am very sorry for you. You write very well and sound like a lovely person, whose life is pretty good ..... but who is allowing themselves to be abused. I know that sounds shocking, and I hope you give the idea due consideration.
The general tone of your 'explanation' revolves around his will, what he will leave. Ae you aware of the likely costs of care. In our current age, it us reasonable to expect that people's estates will be used up in their end of life care. Rather than excuse his behaviour thinking of the party you will throw when he is no longer around, aka allowing his wealth to control you, please consider finding a away to break the bonds while he is here to see it. Is much more difficult but nothing would be more rewarding.
If I don't jump through his ridiculous hoops he pretty much tells me 'well, I'll be dead soon and then you'll regret it'. Instead of caving in at this feeble attempt to blackmail you, how about preparing an alternative response, something like 'oh well, we will just have to deal with that when it happens'. And then do not give into him, use that breezy humour to keep doing what you meant to do and do not allow him to derail you.
It's obviously so much easier to say, than to do. You sound as if you could do anything you set yourself to do. At the moment you are using all that energy to enable him, so how about, well, not enabling him. The penalty would be that he will transform in your mind from the current, all powerful figure into a really rather pathetic little man. You may prefer to be abused by the view of himself he projects.
He knows the rather pathetic version that he is covering up. All the best.