That sounds very difficult to balance around. Why yes let me stick Cloud w/Cover in my team, let him open with a free Blade Beam each fight, and then make it take 2 hits to actually kill him. And Ayla now gets free Dino Tails of MTy Doom and when somebody is faster, it doesn't matter because she takes 2 hits too. Maybe throw Cecil in and whoo yet another person that must be chopped down via auto-Cover before the real targets can be hit. Of course two MT pokes means sure doom for the team if they somehow go off, so... that sealstone is really wacky, since scrubby fights like Crono + Nina3 + Jude are dangerous, as are very tanky bosses who can't be blitzed and have multi-acting MT, but anything else dies horribly.
It's not a bad idea, but having it affect the entire party + Focus Sash that doesn't require full HP effect is kind of egregious. Instead I'd recommend something like:
Crisis Sealstone: At the beginning of each fight, the equipped party member this is on is reduced to X% health as if by enemy damage. Pick X on Floor 1, it can't be changed (though the Sealstone can be moved.)
That still seems decent, and the fact that there are only a few characters it makes sense with isn't as much an issue since it only affects 1 person! It still means initiative limits / turns for FF7 characters, the Ayla dropping Dino Tails at high speed strategy mentioned above, but at least the holder can still be KO'd fair and square.
Re general sealstone philosophy, I agree that there's something to be said for attempting to balance around a "neutral" no-sealstone as the default, the Dungeon's moved away from that and it never really was true. Even the really old inverting Speed? was basically a pure buff that was just limited to going on very slow characters. So yeah, go buff the MT sealstone.
Neph, I note with dismay that Elincia STILL is not a legal pick in the Dungeon.