Wow. what great questions! Really has me thinking. Some of these answers I created off the top of my head, thanks guys. Keep 'em coming!
Is it the pain or the actual damage that is converted to magical energy? It's the pain.
Does it have to be received in combat? Not solely, everyday nicks and scrapes can be channeled into energy, but the real juice lies in combat and extensive training. In training, by the way, they don't pull punches, but there are levels.
What about self inflicted wounds? Self infliction doesn't work, as the person knows beforehand to maintain stoicism. It's the non-reaction to unknowable pain that gives them the power.
What about those inflicted by friends ( a little S & M)? I had never considered that before.. I'd say that wouldn't work, because there's actually a factor of enjoyment to it and kind of dissolves the Stoic power.
When the injuries are converted to magical energies are they healed or is the person still physically injured? They're healed.
What prevents the enemy from simply not targeting these superpeople? The enemies, who are more or less Stoicists themselves, always target with intent to kill as quickly as possible. This has forced the Stoicists to become experts at evasion, but always keep a part of themselves in harms way as to build up energy.
If they know that the more you hurt this person, the stronger they get, why not just order everyone to not hurt them? As stated previously, their intent is to kill quickly, any pain or injuries caused are strictly accidental. The stoicists enemies play it through cats paws, they try to maintain quietness and only fight The Stoicists if necessary or if an order to kill them has been botched.
What happens if you have a one on one fight between two superpeople, does it just go on forever as they steadily get stronger? It isn't their strength that evolves, it's their power. Completely different. For example, someone may be able to lift 200 lbs. But if you've been lifting 130 lbs all day long, it's going to be difficult to lift more. The stoicists battling each other can become a stalemate if both are cavalier with their evasion and pain issuing tactics, both can continue gaining energy but it doesn't mean they're on even ground, there's still experience vs. strength and it's something I still need to develop.
How long can they store the energy, and how much can they store? The energy can be stored for however long they want to, some have even trained themselves in picking and choosing which scars will be used and such.
Could someone essentially do nothing but beat the crap out of themselves for years and eventually take on entire armies alone? As stated earlier, self inflicted pain doesn't work. Just hurts you. I'm actually thinking about it taking away some of the power. A price to pay for the pursuit of power, I guess.