The shotgun RNG makes the first virtually impossible - as it is, shields will trigger when you CAN save yourself by shooting an attacker without moving out of their attack range, sometimes even if the kill is guaranteed. As it is I often have to burn through my shields intentionally before the game lets me fire. As for the second, well, shields are to stop you from killing yourself on YOUR turn, not the enemies' turn. Shields saving you on their turn would change everything. So to get what you're asking for, a shield would have to activate if there's a chance the shot you want to take will destroy an interposer. I wouldn't even want to think about coding that if I were the developer, and even as a player I'm not sure I'd want that. As you note, it's fair, and you know it's fair.
Viewing post in Shotgun King: The Final Checkmate comments
With the first one I was mainly talking about times when I don't have any shells loaded at all, and thus shotgun RNG has no chance to take effect. I'm fine with runs that end on shotgun RNG,those are very rare; however, about a fifth of my runs if I'm remembering correctly end with a checkmate I have no way of knowing about until I look at every possible move. I would rather the game just end there instead of letting me pick my poison.
I'm aware the rng of the shotgun would probably be too much to work around with the second one, but it was just a suggestion. It would maybe be a cool card upgrade for shields to activate on those sort of cases, but it would be a little broken as a general game mechanic, as you said.
The shields activating on the chance of an enemy's defeat by shotgun would be annoying, although one potential way it could be implemented is having the shield activate on a potentially dangerous move, but instead of completely canceling the move it brings up an "are you sure?" prompt. If you hit yes, the move proceeds, and you roll the dice... if you hit no, the shield is still spent, and it goes back to the 'select a move' phase.
Again, just a suggestion, maybe not easy to code, but it would be a good thing for game flow in my opinion.