I can't help with all of that, but here are a few
The high/low is strictly a DPMS thing. It describes how tall the top rail Is over bore axis. It's important when choosing a handguard to ensure that they are the same height or you will have an ugly "step". It doesn't affect functionality. All AR10s use a standard height that is about the same as DPMS high. DPMS low is the same height as standard AR15 receivers. I'd assume your noveske upper is AR10 height so you don't need to bother with that...But you can measure the thickness of the shelf over the charging handle to be sure.
The A lowers are designed to take PMAGS, or any of the other DPMS pattern 308 mags. (3rd party) not the proprietary AR10 mags.
A DPMS upper will not fit your AR10 lower. At all. you should not take any more advice from the rep who told you that.
Your AR10 barrel should work fine in the Noveske upper, but if it's designed to work with the B lower, it may not function properly on the A lower. I'm sure one of our armalite nerds will be along to help with that question.