Yes evolution is loaded with fish, though they look to be small
Yes, expect Mosquitos unless it is a exceptionally odd year
Side trip to Darwin bench, great camp just as you get to the top
No, snow will not be an issue, nor with the evolution creek crossing
I would not expect someone in average shape to have trouble with that timing.
This is one of my favorite areas. There are some great protected campsites just to the north of the outlet in the small grove of trees. If you don't mind cross country, there are a couple of routes from Darwin bench that will take you back to the Piute Pass area.