The Omni MBC I've shot does dry fast but it may be too fast with that reducer.
I'd opt for Medium with those conditions and it will still dry quick
You'll probably need to tack the car after each coat drys as you'll have some overspray and painting over it will be bad news.
Waiting an hour between coats is no problem,makes for a long day but your assured it's dry before the next coat,not that you need to wait this long but FYI.
Sanding out runs or dust nibs is fine but you will need to dust some more base over those areas. 1200 is kinda light for a run IMO, I use some 600-800 wet and clean well after. Like Vince stated,you will have to repaint any metallic sanded. You can try dusting some base and blending it out but it usually ends up being another coat.
Omni used to be a 1:1 so you should end up with 2 gal sprayable out of 1 gal of paint. I don't know what or what all your painting but for just the outside,you'll have plenty, Jambs,trunk,hood,fenders,Nope not for 3 coats.
Clear,20 min recoat is just that, "Should" be ready for another coat in 20 but that is subject just like base to be longer.No scuffing required.
24 hr,is basic complete dry time to buff or sand for recoating. Again, subject to change.It may take longer for the clear to dry enough to sand or buff.
Try some 1500 dry and if it balls or gums up, it's still green.
I would not apply more than 3 coats of clear at one setting. That should be plenty. If it's looking rough after 2,stop and let it dry overnight or the 24 hr. and color sand it with 600-800 and reshoot it after you figure out "why" it looked so bad. You'll either o'peel the snot out of it or dry spray it.
As for tip size. I prefer a 1.4 for either base or clear. Choose between a 1.3 or 1.5. I'd go 1.3. You'll need to do a lot of test patterns to get the gun/air right.
You "generally" run about 10# more for clear.