Hmm, I can understand your anxiety.
I would say you might need at this point to consult a solicitor on your own, to discuss what your options are. It has, however, always to be borne in mind that when we marry, we 'commit' our financial future, to an extent, t our spouse, in that they then become legally involved in our finances! (I personally hugely disapprove of marriage changing our legal and financial status, and strongly believe that 'pre-nups' or equivalent are essential to every marriage - property and money are 'business affairs' and should be treated as such, as if we were going into a formal 'business partnership' with our spouses!)
So, for example, if your husband decides to divorce you, he might be legally entitled to a 50% share of a house into which you have put more money than he has...(etc etc).
However, presumably the immediate issue is whether (a) your husband is deemed still to have legal capacity even with his level of MH and (b) what would happen to his finances if that changes (ie, he becomes 'incapable' due to a worsening MH).
And, as I said above, to discover what would happen in the event of (b) were your husband to include you in his existing PoA granted to his mother (ie, what if you and your MIL disagree when you share PoA together).
But I definitely would suggest that, irrespective of what you find out here, either from other members, or the Carers UK helpline, that you get that verified (or refuted!) by a solicitor acting on your behalf, to clarify to you your options, and what the law would decide, were your husband to be deemed without legal capacity.
It's a grim thing to say, but one possible option for you might be to say you will divorce him now (ie, to retain your fair share of your joint assets), unless you get full PoA (to protect your and his finances)(maybe the health aspects of PoA could still be shared with his mum, though it sounds like you have different views on medicine anyway, from what you say of your FIL) (That said, as a mum myself, I would hate to think of a DIL doing something to my 'incapable' son that I profoundly disagreed and disapproved of - my maternal instincts would be very worried!!!!)
But it's definitely a tricky situation! Does your husband tell you why he doesn't want to remove PoA from his mum? Does he have a 'good' reason?