You have so many questions here I can't answer them all at once. If I were you, then I wouldn't join the Navy first if you are wanting to go to med school. The navy will help you do it, but it will take you so many years to get there that it may hinder your chances of getting into med school. med schools
want someone who is driven, hard working, and focused. If you take classes here and there from multiple different sources it shows inconsitency to admisison boards, it shows that you may not be as focused as someone who got a 4.0 in your undergrad in 4 years. Under grad & Grad schools may also not accept some of the credits earned if you take them from the shipboard professors. BTW the professors on the ships tend to only teach the very basics, lower division math, english, history. Plus the credits earned may not be recognized by some schools. I could go on about this.
Regarding your HM rating helping you with med school, maybe. Many of my corpsmen are amazing and smart and could absolutley become a physician. However I don't think that becoming a corpsmen will help your chances as much as you think. What I know it will do is that it will give you confidence when you start working with patients years down the road. You won't have that scared or hesitant look on your face when you are doing an exam or an assessment on someone. What a HM rating will do for someone is give them the basics to succeed in nursing school
or PA school. Also obtaining a rate of HM will allow you to test out of the LPN/AD NCLEX depending on your rank and years in.
Overall I applaud you for your desire to want to serve your country however I would really reconsider how you go about getting to med school. There are programs that will help you through undergrad and med school: ROTC, HPSP, etc. These things will help you get to becoming a MD sooner, and you will still get to serve plenty of time in the military during your payback for these things. Good luck.