The cheapest solution will be to agree on the estate planning whilst everyone is still alive and able to have a conversation. Things only get expensive following a death when everyone expects the will to say something different, and then they fight about it. With lawyers.
I suspect you don't want to discuss the plan with SIL, though, because you know your proposed split is inherently unfair, and she'd be perfectly reasonable to get upset about it.
From an IHT point of view, it makes no sense for either parent to sign over their house when they're still living in it due to the GWRB rules. Unless, of course, they charge you market rent, and you pay it (but it doesn't sound like you'd want to do that).
Deprivation of capital is not my area, but I suspect you might have an issue with trying to hide housing capital to wriggle out of care home fees. At best, you'd be in a situation where FIL or MIL could end up with the worst possible care due to a lack of funding - you may be OK with that because they're not your parents, but your DH really shouldn't be.
You sound a mixture of resentful and entitled, and I think perhaps the living situation of living with your DH's parents hasn't really worked for you, and hindsight is a truly wonderful thing. Ultimately though, you went along with this scenario where you accepted someone else's parents as your own, with all of the faff of living with aging parents that that scenario entailed. There's no compensation morally or legally due for tolerating the annoying behaviour of family.
If your DH dies before your FIL and MIL, I expect you will be out on your ear. Your DH, therefore, should have adequate life insurance, so you and your DC aren't financially screwed following his death.