That's the standard (nominal) measurement - just subtract 1/2" from any lumber "size" for the actual measurements...
Wood shrinks and expands with humidity and it's not uncommon to see a 1/16 to a 1/8 variation when lumber changes climates
No - use treated lumber
It's not recommended to use 2x4s for rafters, except over extremely short spans. Even then, 2x6 is better. Also, if you're going to be cutting them to length, it's better to buy regular 2x4s, not precut studs.
A bunk of 2x4s is usually 294 studs. Prices vary, according to location.
You're gonna have to cut / combine pieces of nominal measured wood.
This is usually due to lack of sales of that particular item in your store.