Finn

That the mercs have to try the keys one by one would actually be more realistic in my opinion.

Maybe. It is mindless make-work to f*** with the player though. If I tell a merc to unlock that door, I expect the minimal intelligence feat of trying one key after the other until one fits or all are tried. Allocate a certain chunk of time per key and add the option to try a certain key manually if you know that's the one and don't want to wait. Tada - well rounded system.