Your shock level right now is 15 ppm...you could probably go up to 20 ppm just before you leave without any problem other than possibly the fumes fading your cover a little, but if you leave it vented it should be fine. (I don't use a cover, but I'm sure someone here who does will pop in and advise you better there.) How confident are you in your in-law's testing abilities? If they could test, and get it back up to shock level, that would be great. If not, then having them pour another gallon in (which would raise your Cl by 12) should be sufficient.

In looking through your thread, I don't see mention of a pH or calcium level, but another option could be to purchase a floater and put a couple of trichlor (if you can withstand a lowered pH and slightly raised CYA level) OR cal-hypo (if you can stand a slight calcium increase) pucks in it, then tie it off somewhere in the pool where it can't float over and make contact with your pool walls. This would at least allow for some continuous chlorination, but honestly I'm not sure how well it would work with your pool covered. Again, maybe somebody here that uses a cover will comment on that. You do need to take a look at your pH level, though, even if you don't use the floater--you don't want a pH lower than 7.0 because it can start causing damage to your liner. Keep in mind that pH at high chlorine levels will read falsely high, so if you test it while the pool is shocked, and only get a 7.2, then I would for sure add some Borax to adjust it upward.

Honestly, don't lose any sleep over this--I'm sure that by shocking the pool before you leave, and having your inlaws add a gallon of bleach mid-week will be fine--the absolute worst thing that could happen is a little green when you come back, but that's easy enough to take care of in a few days.

Janet