I'll be darned if I can remember which month I was last there, but I do remember that the mosquitoes were not much of a problem at the Iva Belle hot springs. (which, by the way, are wonderful).
As I vaguely recall, the mosquitoes got really bad after second crossing heading south at the bottom of the valley, which is all pristine meadows and beautiful cascades (thus, the name). But the trail is basically next to water the whole way, so there are lots of mosquitoes in season, (and an amazing number of trout).
The hot springs will have some traffic, as it is basically a day's hike from Red's Meadow. There will be some traffic in Cascade Valley proper, but not much. In the hanging valleys where the lakes are located, you will be lucky to see people.
I wouldn't camp right at second crossing, unless your logistics work out that way, as there is wide-open camping all along the valley floor, that is much less populated.
