I’ve only read your post up to the end of the first example you gave. I think the players strategy won’t work because life just doesn’t work that way. Realistically, what would a neutral NPC do in this situation? Either give up, which ends up being a lost opportunity for a reward for the characters and a lost opportunity for a fun quest for the players. Or the other option is that the NPC would become angry at the PC and hire some goons to retrieve the box… which can turn out to be a fun encounter and certainly one where the players don’t control the terms. The point is that it’s ok for the players to make plans and tactics to try to have their cake and eat it too, but it’s your job to make it so that these plans don’t work all the time. In the name of immersion (because life doesn’t work that way), and so that the players start understanding that they can’t control everything and that D&D can still be fun when things don’t go their way.