1. There's no such thing as too hot. Temperatures over 100 F can occur in any of the summer months, and you can bike through it anyway.
2. There's no such thing as too cold, as long as you dress for it. There is, however, such a thing as too slick and too closed. If you're intending to cross the Rockies or the Cascades in May, you're going to need a lot of luck, probably too much luck.
3. Assuming you pull the addenda, it's as accurate as they can make it. However, you always risk the restaurant or motel that closed yesterday. It seems to occur more often than you would think.
4. There are always a few segments where you might go 60 miles or more without services. You will be aware of those segment ahead of time, and you can carry more food and water.
Have you added up the total distance of the route you are planning? Can you really do it in six months? Are April-May and July-August the only two windows you have available? I.e., is June out of the picture?