1. Does it cover the essential things or am I missing something?
Looks good. Just don't forget to think about the future. If there's any location that you might want a speaker, run the wire now. Same for Cat cable. Cat cable can be used for all kinds of things, so it doesn't hurt to have a few extra runs hidden behind the walls.
2. Is his pricing reasonable? Is pricing with builder contracted dealer negotiable and what's the room?
Hard to tell without knowing your location and the particulars. There is little negotiating you can do. You either wire now or wire post construction and pay double (or triple) the cost. He knows this.
I have seen mixed reviews for speakercraft AIM8 ONE speakers and considering that they are entry level speakers in AIM series, should I go for prewiring only and get in ceiling speakers installed later from a separate installer with me providing the speakers?
Don't get them then. Have him run the wire and you can get the speakers later on. You can pick up a pair of speakers for under $100. Or, if you really want to go high end, you can spend over $1000. My suggestion is to start with decent 6.5" speakers and upgrade later if you want. Easy to go from 6.5 to 8... not so easy to go the other way.