I feel for you. I don't think I've heard of such a young joey with a broken leg before. So you are breaking new ground here.
Spryte was an adult glider when she broke her leg.
I suspect that young joeys are like other young animals and their bones are still fragile as they are still growing. Like with my puppy, the tumble her mom gave her broke her leg but most likely would not have had she been 4 months older when it happened.
It is just so hard when there is such limited experience dealing with a situation.
I can't advise about the HPW as I don't feed it, nor have I fed it to young joeys (hand raised or just barely weened). I do know that there has been many voice their concerns about the calcium content but with older gliders, that is balanced out by feeding certain fruits/vegies (the ca:p ratio). But with babies like your's, they may not be ready for those fruits/vegies.
Adding oral calcium can help the bone rebuild but again, to much can cause them to over calcify and we don't know how much is to much.
There have been very young joeys on the antibiotics though. And pain meds...if the vet
can calculate for a 100 gram glider and feels the joeys needs 1/4th, then just dilute with juice (or water) until you reach the amount per drop that is needed. So if an adult would get 1 drop, take that one drop and mix with 4 drops of water...then one drop of that solution would be appropriate for the joey. I'm sorry, I have a hard time with vets
that can't figure that part out. (and your's wouldn't be the first!)
I also worry about insulting Dr. Tebay. He is seriouslhy the best vet around here and i don't want to make him feel like i don't trust him by asking him to consult with another vet.
A good vet
is going to do what is needed to help the animal placed in his/her care. Consulting with other vets
should not be insulting. It is a learning opportunity for your vet
and one he should be willing to take advantage of, especially if he is dealing with something he's never had to deal with before.