So if AppleCare+ ends and the keyboard cannot be fixed by replacing individual keyCAPS, then we need to pay a lot of money to have the chassis replaced?
Yes, the same as every MacBook Pro ever made.
Yes, but I wouldn't worry about the keyboard on this having a problem. It's a proven keyboard design that's been in use in other products with slight modifications, not an entirely new, unknown keyboard.
Don't worry about the keyboard anymore.
But also don't forget that scissor keyboard fails too. Not because it is bad, but because everything fails. By now, its pretty clear that butterfly (especially the earlier designs) is more prone to failure (by a factor of 2-3 according to the only empirical study I am aware of), but it doesn't mean that scissor failure chance is zero. We have a pile of broken magic keyboards in our cellar, as a matter of fact.
People have been so caught up in all this butterfly fiasco, that they forgot the basic facts of chance. Your keyboard will not fail just because it happens to be butterfly design, and your keyboard is not safe just because it uses scissor design.