Your husband is in the right place. If he graduates with his NEC and gets pick up STA-21 he can make rank as well while being in college (he'd be capped to E-5, then as long as hes not an electrician, he should make e-6 pretty fast. While not important, it would be nice to have that under your belt).
1) He would be kept on hold while waiting for his results.
2) I had crap highschool GPA, I looked around and applied to a few colleges (purdue, U of M, and Mi State for starters) and was QUICKLY accepted. That really wont be an issue for him.
3) Yes, but no A-school obviously.
4) Prototype is nothing compared to being out in the fleet. It may seem overwhelming, but thats just how it is. If he;s having difficulties coping with student life, he may be wise to question himself as an officer in the fleet (feel free to chime in gamecock). They have a lot of stress the enlisted boys (at least the <e-7 types) dont have to deal with.
My 2 cents -> If hes going to do it, now is the time. Hes in a great position, hes young, hes still fresh in the navy, so his ideologies havent been smashed yet

. If hes up for it, I suggest he take a peice of advice given to me late in my naval career "Take everything you can get from the Navy". I think STA-21 is a great way to do that. Not to mention, watch officers are worth oh so much more then us blue collar working types