Lele,
I agree - a close look at their environment is in order.
However, keep in mind that if all chameleons were to survive (and for long periods of time) - with no casualties - explained, or otherwise - there would be areas over-run with the critters.
In the wild, they are destined to be a link in the food chain - either when healthy or sickly. Mother Nature designed these creatures to fight for every day's survival. First, they have to dig out of the ground. Second, those who do make it, are food for waiting predators. Then the ones that avoid the endless snakes and birds who would make a meal out of them, have to find food - find a mate - and grow to start the next generation.
Their life is a tough one, and we are doing our best to give them paradise - a safe haven with regulated rain, endless food and optimum temperatures. If we succeed, they fill our days with joy as they live their "captive" lives.
Since it really isn't documented how long that life should be (in paradise), we can only keep trying.
One thing I recommend - fresh air - the air exchange makes a lot of difference. All of our vivariums have small screened windows in at least 3 sides, and on the top. This allows the plants and terrain to dry out in between mistings - and in their natural settings, I would consider that to be normal.
All of the breeders experiment to a certain degree with environments for each the cham species in our care. Once we reach a certain success level - we sometimes get a false sense of security. Even when most of the animals are responding favorably - reality presents you with an exception. They are not clones - they have their individual needs - and the tiny leaf chams can hide their distress very well until too late.
You tried very hard - heck, we all know that - but it just may have been some wierd tiny thing that was overlooked in the husbandry (although from all your descriptive posts - I can't think what it might be). Then again - it might not have been related to that at all. None of us are experts - no one can know for sure. Even with a necropsy, it could be something hard to pinpoint.
Take good care,
Morgana