I'm not an expert, but my 2 cents as I understand it:
1. I don't believe so. But keep in mind that A+ in unlimited. A super super high A+ has more advantage than a just barely A+. I believe it was cbg that has posted about it being a scale (I'm sure others have, but he comes to mind), so an A- might be a 90.1, whereas a B+ might be an 89.8 for example. I've understood it as figuring the difference between letter greades and applying them using whatever formula from number 3 below, but keeping in mind that the letter grade isn't necessarily exact since there will be high C+, low A+, etc.
2. From everything I've seen, no. At the very least not at D3. The biggest advantage for prestige at those levels is the recruits available immediately, and those that can be pulled down. An A+ prestige school at D3 has
much better recruits available to them than a C+ team, even if there's not necessarily a multiplier to it like D1.
3. There are formulas that some coaches have published with some wriggle room, but this is one of those closely guarded secrets. I found an older thread that had
lostmyth's guide where he says 70% per letter grade. some people assign other multipliers.
You may know all of this, I'm far from an expert, but that's what I understand now and hopefully other, better coaches will pipe up now :)