So I had always heard that wearing a full suit of armor gives a "hidden 100 Point Bonus to AR", but I just read on uesp that it is 25 pts per piece. So which is it? Do you have to wear a full set to get any bonus or do you get an incremental bonus for each piece?

Another question is whether or not Bone Breaker ignores a percentage of any hidden AR bonus that NPC's get. Or do NPC's get said hidden bonus.

Both, kind of. 25 points per piece of armor for a total of 100 points when wearing a complete set. Not sure how NPCs and Bone Breaker work.