You have a builder, so I am confused as to why you are asking these questions. Yes, 24" is not only safe, but is code and is what you will see in most new construction. Prefabricated roof trusses are normal and common, and the savings comes from the labor of assembling a truss or rafter system on site. Certain designs still demand onsite construction of certain roof components, without seeing your plans, it's hard to say.
Drywall will not "sag" as long as it is installed per code. I will assume your builder will have a professional drywall sub do this for you, but in any case, the drywall should be installed perpendicular to the trusses, so it spans as many of them as possible. The drywall should be 1/2" or better, and should be SCREWED to the ceiling in a manner consistent with the building code in your area (typically every 6")
I am unsure as to what the roofing material will do to "hide" the waves, your roof sheathing will do that, if it is installed correctly. A 7/12 pitch is pretty danged steep, make sure you have a LOT of tie off points installed, or you will NEVER get anyone to clean your roof later.
The only concern I might have is if your roofing material is tile or slate, or something else HEAVY. That could affect how the structure of the home is built, but these are issues best resolved with your inspector and permit reviewer.
Don't think you can change to a heavy roof material after the permit is approved. I worked for a contractor many years ago who accepted a clay tile upgrade on a 6,000 sf house, then spent 4 weeks rebuilding the home from the inside out because the roof made everything sag and buckle. (fortunately, he was only in the framing stage. Just imagine with drywall installed!)
Trust your builder, it sounds like he knows what he's doing.