Shocking that they don't know for sure. Good pet shops should know about their animals and ideally breed them themselves, something I'd like to see more of.

I'd watch with the supplements, not to over do them. Most standard supplements if you use every feed with tree frogs might harm them, once a week for adults or twice for subs should do. I use Repashy's Calcium Plus LoD for my Greens everytime and have had good results but this is more balanced out so it can be used every feed.

Just cut down on the misting if it's going to high but these hygrometers can be really inaccurate in my experience and your eyes might be your best bet. As long as the tank doesn't look wet then you should be fine, with most frogs keep them moist but not soaking wet.

I'm not sure if that will cause issues but I'd say you're just overthinking things and I'm the same at first but after a month you'll start to relax. I'd recommend AVS popular tree frogs if you're interested in a good read about this species.