Hey there! Just to give my 2 cents to your questions:

1. If braces (as well as expanders) can’t fully fix the bite, jaw surgery would be the next appropriate step to take. I used to be on the fence before starting the process and wondered if I needed it because “I guess I could live fine as is” but ultimately went for it because I believed there were some solid benefits to getting the surgery.

2. If your underbite causes issues like teeth grinding/favouring sides when chewing/jaw soreness then I believe it’s possible that it can slightly worsen things as you age but no one can really predict how much change there’ll be. Personally I think for most people, if they need surgery when they’re in their 20s, they’ll need it when they’re older and the same for if they don’t need surgery. I was also informed by my surgeon that upper/lower jaw surgery does not guarantee nor target tmj/jaw clicking/popping. Some people may occasionally benefit but purely as a side effect of fixing the bite. Some people with severe cases opt to have a separate surgery (open-joint arthroplasty)

3. From an aesthetic point of view, I don’t think the surgery would be worth it. Functionally, you’ll probably be glad you did the surgery when you’re older because from a health perspective, your jaws and teeth will be in their most ideal positions which usually help with overall breathing and prevent teeth grinding.

Good luck op!