Veiled Chameleons are the most robust Chameleons around they can handle the widest range of temps and humidity of all Chameleons, there are now even wild colonies all over Florida so I doubt long term humidity near 100% would be any problem for them.
Stress from handling is the same for all Chameleons and is really an individual thing and with trust and proper handling this should not be an issue. In fact I've seen and read they are actually more social than most Chameleons and will tolerate having multiple Chameleons in one enclosure and...
You should get a Trioceros sternfeldi or commonly miscalled a Rudis Chameleon they would be fine with the cage you have but would need more plants and misting and are a better Chameleon
Try hand feeding the veggies 1st make sure they move and wiggle. That is how i got Doughnut to finally eat veggies instead of munching on her Pothos. At 1st she would not eat any veggies in her food cup now that she recognizes them from me hand feeding her them she now eats them out of her cup...
Give him some worms silk, horn, and butter. They will help hydrate him mostly the horn, silks are good for vitamins, and butter will help gain fat. Still dust with calcium with out d3.