Hi there,
I defer to the experts here but can offer some advice from my family's experience with it so far.
My son is a ET student in the Navy nuclear power program now. I would not worry about just one waiver for the one math class you got a D in when you were 14. If your other grades are good you will get in. You may have to wait a bit for NR to process your waiver from weeks to a couple months ... well that is not a big deal either. Don't worry about the NAPT either my son had auto-qualified for nuke well beyond 252 and had a 99 AFQT with no academic or other issues and they asked him to take the NAPT test anyway, he got a 69. Get used to taking whatever academic/physical/medical/promotion exams the Navy tells you to take, it is all good and for your benefit as long as you pass it. A lot of DEPpers seem to fixate on having to take the NAPT not sure why.
I asked your same question in the forum a while back out of curiosity and Yes there were some people who enlisted as non-nuke and then got accepted to nuke before boot camp. So it does happen. Here is the thread if you are interested:
http://www.nukeworker.com/forum/index.php/topic,27173.0.html Just remember the final contract you sign on the day you leave for boot camp, is the best you will get. Also, a person can be dropped even during boot camp but they do not add nukes.
My son will be graduating Power School this Friday in the top 1/3 of the graduating class but still has a long road ahead of him.
Good luck to you, you will do fine.