Hey, Jen, welcome to the Forum. Sounds like you are doing very well with your frogs so far. I, personally, do not have Gray Tree Frogs, but we have an excellent caresheet to the left that might help you answer a few of your questions. In the meantime, let me take a crack at some of your questions:

1. Young frogs have high metabolisms and fast growth, so they need that extra food to kep them going. Seeing them fat and then skinny is entirely normal. However, as you've noticed, once they reach adulthood, we have to adjust their food so that they don't become obese. I raised 5 Pacific Chorus froglets, and while they were small, they were fed every day as much as they could eat. Now, being almost full grown, they are fed every other day (3 times a week) 3 crickets each. I would still offer your chubby frogs food every other day, but only one or two crickets of appropriate size until they slim down some. If necessary, separate them to ensure that is all the food they are getting for the time being!

2. I have never hibernated any of my frogs or toads, but I know that some Forum members do in order to keep their frogs on a natural cycle. This is especially important if you plan to breed down the road. If breeding is not high on your list, it would be a personal choice between you and your frogs.

3. The setup for my PCFs is very similar to what you have. I find this setup to be easy to maintain, but I know there are a million other options out there. There are a lot of members who have a secret talent in doing setups and hopefully someone out there can help you. Check out the Vivarium section of the site as we highly encourage pictures to help out everyone! Best of luck!