if you ask 10 times, you'll get 10 different answers on best rust prevention.
I found that POR15 would probably work, but you really need to spend the time to prep the surface. It doesnt hold up well if you simply paint it over the areas, whether rusty or not. I can confirm that less than 6 months in I was able to literally peel it right off of the areas that didn't come off already themselves.
What I have found after that seems to have decent overall opinions on is this blaster surface shield:
Amazon.com: B'laster Surface Shield Complete Corrosion Protection for Long Lasting Automotive Undercoating Applications, 12 oz Aerosol, Pack of 6 : Automotive
Have yet to spray down the frame with it but know folks that have used it and swear by it.
regarding the broken swaybar bolt.. that's crappy.. going up a size would work, in theory, but you still need to get both bolts out. My only concern would be tapping the one hole closest to the support member since you won't be able to get fully turn the handles.... maybe if you used some sort of an extension .. but you get the idea. I've gone up bolt sizes all the time to fix similar issues.
It could be easier to try and carefully drilling out the bolt so you don't screw the threads up too much and then just chasing the threads to clean them up, but that depends on why your second bolt is spinning.. something is stripped .. either the bolt or the threads in the frame.