Q12: What is the general opinion of taking Involuntary Rage and Pain Mastery (both in SS) on a Barbarian?
Don't know about a general opinion, personally I don't think it's such a great idea. You don't have a whole lot of feats to work with, and you're blowing 3 of them for the ability to stack a basic rage onto whatever other rage you've got going (if you take one big whack and not two moderate whacks that add up to the same or more damage), and the ability to add a (maybe cumulative) +2 Str for every 50 points closer to death you get whether or not it comes in one chunk.
If you don't find the feat-starvation bit problematic, that might look better on somebody who lives to get the crap beat out of them, and can reverse its effects before dropping dead (the feats only say you need to take the damage, not keep it). Have you considered trying the same combination on a Crusader?