In order of the stick damage amounts and enemy hp always seems to be pretty handwavy to be whatever the story requires, and not always even remotely realistic for what the players are and who they are fighting. So I wouldn't let that worry you too much.
On some barbarian stuff, the con bonus from raging applies to all levels, although more properly it's all HD (although characters with HD that don't match their level are pretty rare) since you gain your con bonus times your hit dice in health naturally already, so that's really how it works, +4 Con means your Con bonus is 2 higher, so you get 2 more hp per HD you have. (Also ninja'd by Rolan. Although to Roland, when it says they don't go away first, that means you loose that much hp when you stop raging, unlike temp hp, which if you have 10 temp hp, take 10 damage, then your temp hp spell runs out, you don't loose 10 hp because the 10 you lost were temp. With a barbarian, if you have 30 rage hp, get knocked down to 10 hp left, then stop raging, you instantly go to -20 and die.)
As for activating rage well unconscious, you can't do that as a normal barbarian, it has to be something you actively do. Which is sorta why I think he wasn't actually unconscious, just dominated into "staying down". However, there's of course plenty of ways (of course there is, this is 3.5, game of 1000 splatbooks) you could in fact activate rage when unconscious if you weren't a normal PHB barbarian. Alternate class features and prestige classes and such. So that's a possible (although imo unlikely... I think more unlikely then just "it's not part of the rules, but just part of the story") answer to how he got up.