Firstly, I would remove the gravel and the moss - the frog will definitely get the gravel in its mouth and probably swallow some at some stage, and this can be very dangerous. The moss has the same risk, just less likely to cause problems.
Answers to your questions:
- Frogs are Amphibians, not Reptiles. They do not need a UVB bulb, nor do they get any proven benefit from it. I use incandescent bulbs with these frogs to raise the day temperature above the "base" temperature provided by a heat mat over part of the bottom of the terrarium (or at the side, depending on how thick the substrate). I give all my frogs 12 hours per day, usually using a timer. Pacman frogs hide all the time any how, that's how they're wired. They only come "alive" for food.
- Sounds like you are not keeping the frog warm enough. The way to get the frog to eat well is to keep it at a healthy temperature and if you want it to eat from tweezers, don't offer it food any other way. For young pacman frogs I would dust with a calcium supplement once per week. Earthworms are a great staple for these frogs.