could you leave them a printout of a VERY simple translation of the legal language on the top, with the actual legal language printed at the bottom? or maybe you could mail that to the corporate people, along with a short explanation of what's been going on? the corporate people might not realize what the store staff is doing, and what kind of outlay they could be facing if someone's laptop gets stolen.
if they take to your raffle ticket idea, is there a way you could lock your bag up so no one can open it? maybe bring your own crushable bag that can be locked with a padlock, and put the backpack in it. either you could leave it with them or maybe they'd let you carry it if you give them the key. or maybe you could carry all of your stuff in one of those shopping baskets. just dump it out and give the bag to the people. sorry if that's a repeat; i've read this thread for so long that i don't remember what has been suggested and what hasn't. i know you shouldn't have to do a bunch of weird crap like that, but if the point is to keep your stuff safe without having to ride to a supermarket 3 hours away, then it might be a good compromise!
or, i dont know. maybe you could lock your bag TO something. either use a cable lock to attach it to the manager's desk or whatever, or get a lockable trunk or something like that on your bike and lock the hell out of that thing.
i agree that there's a point that this kind of fight gets to be way more effort than it's worth, and causes more problems than it solves, but when you don't have a car and you have to have a safe place to keep your valuables (especially now that the people at the store are pissed off), and there's no accomodating store to shop in, you have to figure something out. i live in a smaller town than you do, and in your situation i don't know where i'd go if the two grocery stores (one of which is tiny) didn't care about my business. it isn't like a big city where you just go someplace else. so i feel ya.