3) I have not heard of grinding the seat area of the body. If there was a crack, I don't see how this would help. The crack should be visible under a nice bright LED torch, and the area cleaned well.
I have seen folks recommend reinstalling the seats with JB Weld, or silicone. I have never done that, as it would be a "permanent" and possibly "fatal" procedure. If the body is really cracked, you will just have to bite the bullet. But that is rare. I would definitely take it all apart ( both hot and cold) clean everything well, and use new washers and seats "just because". I do use a very narrow wrap ( 1 turn) of teflon tape. Not all agree with that. Be very careful the teflon is not wrapped over the top of the seat.