I think you have to drop Stassi for a backup infielder. Only having one backup for 3 positions just isnt enough, if one player is slightly injured you have to make a move. One could argue that Herrera could be that guy, but he hasnt played 2B in a few years now, not sure they could count on him for 20-40 games filling in at 2B over the course of a season, but maybe try that.
Stassi is 28 and really has done nothing to prove he needs a ML spot. He is a AAAA 1B whose best skill is 1B defense (pretty negligible) and really doesnt hit enough to justify a bench spot. I would rather have the veteran take that spot, especially if you are talking about catchers player 1B also. He can be held in AAA awaiting an injury to fill in at 1B, OF or just as a hitting bench player, which is pretty much the definition of a AAAA player. If he moves on to another team he will be in the same role, there are no teams out there that would trade for him or offer him a ML contract.
I think you do need a veteran in there somewhere, and I am certain that Pete will fight for it. Additionally, it would be nice to have a trade chip again. Williams really is not a lock to start yet, he has done well and had a nice hot streak in AAA, but he still needs to prove that he can hit ML pitching once they realize he has bad plate discipline.
The 3 catcher thing is just because that is what we have available rather than actually smart. That is a great deal of "slow" for one team. I would rather the team make a decision and trade one of them, I would look to move Rupp and get a reasonable return for him, but really any of the 3 would be acceptable to move. It would be best if we could get 2 high prospects, preferably not 40 man eligible. Former 1st or 2nd round pick types (that havent been failures). A contending team needing a catcher would be wiling to pay that price for a plus hitting, plus fielding catcher in his prime. Rupp is underrated by this board, he has a .750 and .747 OPS he last 2 seasons, that is well above average for a catcher and he has developed into a solid defensive catcher by my eye (above average at throwing out runners, etc.)