I personally wouldn't worry for the fasting...when I first got my bettas, it took them 3 weeks to finally eat pellets. If you decide not to feed them, then I would suggest not feeding him the usual amount when you return from your vacation...you should ease the feeding gradually to his full amount. (sort of like when people fast, they don't eat heavy meals right away...)
You could try giving your caretaker a limited supply of food...maybe 3 pellets a day or something so they don't screw it up...sometimes people keep feeding them more because the bettas "still look hungry." you should explainto them that their stomach isn't very big and they can only ______ a day.
As for the water change, i woulnd't stress about that, maybe change your water the day you leave and put amquel in...
does anyone know if amquel works if there's no ammonia?...ie does amquel stick around to be useful when fish produce waste?