2. As the water temperature changes, fish behavior changes. Lower temps mean fish metabolism drops as well, and their need for food lessens. Warm water holds less dissolved oxygen and the fish will seek out spring seeps or cover behind rocks in fast water just to stay alive. The sweet spot (roughly 45F to 68F, depending on species) is where I usually expect trout to be the most co-operative.
3. Depends on the river. On small mountain streams where food is at a premium, fish will often come up for for almost anything that resembles food, regardless of what’s actually hatching (or not hatching) As long as you’re stealthy and the fish aren’t spooked that is. On rivers with heavy bug life (spring creeks, tailwaters) I’ve found the need to be quite precise in matching my imitations to the prevalent hatch.