1) Biggest question is do great weapon buffs work with polearm buffs, particularly the Great Weapon master and Polearm master feats… do they stack?
I believe they do ...
Whats even better is that GWM +10 damage bonus works with 1d4 Polearm Bonus Action ...
The only let down is that when you kill someone, or score a critical strike ... you dont have any Bonus Action left for that another full attack you get. :-/
Problem is that barbs don't get a lot of feats
Are we talking about ASI?
Bcs if so, that is not necessarily true.
Quite honestly, playing unarmored Barbarian in this game makes no sense ...
Allready in Act 1 you get Medium armor that gives you 18AC, and some other benefits ... wich is something you can barely achieve on level 8 ... and only if you dump everything else.

There is no problem to squeeze at very least two feats into any build ... 3 if you are willing to drink a potion after each Long Rest.
You can get gauntlets that set your STR to 23 early-mid act 3, so you don't really need ASI down the road.
Or potion that sets it on 21 ...
Or another potion, wich sets it on 27 (if i remember that corectly) !!!