Oh boy! Lots of questions!

I’ll do my best to answer them.
As far as I know, gout is not a death sentence for chameleons. It is treatable with medications that will decrease the uric acid (which causes the gout) and anti inflammatory for pain. Diet may also play a part, although for reptiles
@kinyonga is better suited to answer how. Gout usually will show up/be diagnosed thru blood work and radiographs, both of which you should have your vet do. If you just recently added the uvb, the weakness is also most likely some metabolic bone disease. While any broken bones or deformity can’t be reversed once it occurs, it can be corrected with correct uvb and supplementation. It will take time and perseverance to overcome both of these things. If you have the willingness and resources, there is no reason that I can see that euthanasia would be an option. If for whatever reason (no judgement here) you aren’t able to put the time, money and work into your cham’s recovery, I would suggest rehoming before euthanasia.
Balancing the vitamins A & D is as easy as using appropriate supplements on the schedule appropriate for them. The calcium without D3 should be given at every feeding. Calcium with D3 should be used one feeding every other week, Multivitamin should also be given one feeding every other week (alternating with the D3). I use Reptivite which has preformed vitamin A in it. Depending on if your girl has mbd and the severity of it, your vet may prescribe a special supplement regimen.
Regarding eye health and problems, this is a great podcast to listen to…explains a lot. It’s episode 66.
https://chameleonacademy.com/chameleon-academy-podcast-2021/
Not all exotics vets are very familiar or up to date about chameleons, which is understandable. For the best husbandry guidelines that are the most accurate and up to date, follow what Chameleon Academy says.
https://chameleonacademy.com/chameleon-basics/
You can use a 10.0 uvb. You’ll just have to raise it a few inches so that it’s about 10-12” above basking area. Always provide lots of live plants so your cham can choose to take shade from the heat and lights. While I love YouTube as much as anyone, there’s few that I trust when it comes to care guidelines. Neptune the chameleon is the best.
I think I’ve answered at least most of your questions.