Barbarians aren't the easiest class to play, so don't worry if you're finding yours a little tricky. To answer your questions:
1. Perhaps somewhat counter-intuitively, the typical recommendation for a Barbarian is Constitution 10. You have the highest natural Health and Endurance, so you don't particularly need to raise Constitution higher, but lowering it is also a bad idea since the reductions affect you disproportionately (-X% of a large number is more than -X% of a small number). There are Barbarian Builds that make use of both very high and very low Constitution, but they are always trying to do something unusual. As for Might, high is absolutely fine.
I think if I were going to build a typical, AoE damage dealing Barbarian, I'd go for something like
High Might to increase damage, high Intellect to increase the area of effect of Carnage, and reasonably high Perception to offset the Barbarian's lowish Accuracy and the even lower accuracy of Carnage attacks. Having such a low Dexterity might be a bit painful, but with Dual Wielding, Two Weapon Style and Frenzy (along with some other attack speed boosts) it should be possible to attack very quickly even when wearing decent armour.
2. In PoE you don't get any more Attribute points after character creation. There are a few one off opportunities to raise your attributes through quests, and plenty of items raise them too, but other than that what you start with is what you get.
3. The basic rule is this: you can only ever have one bonus of a given type from gear, and if you are wearing multiple items that give the same type of bonus you'll get the best one (for example, if you're wearing a Breastplate that gives you +1 Might, and a Helm that gives you +2 Might, then you will only get the bonus from the Helm). The exception to this rule are Weapons and Shields: these can stack on top of other bonuses of the same type.
Roughly the same rule works for buffs provided by spells too, although that's a lot more complicated and has more exceptions. But generally speaking if two different spells raise the same attribute then you'll only get the highest of the two etc.
4. If you feel like you're doing enough damage then sure, why not. Later on into the game you'll probably be able to do so AND still attack with 0 recovery (the fastest possible).
6. Yeah, there's relatively little benefit from dispositions like stoic etc., there are a few pieces of unique dialogue but generally I think it's something Obsidian thought up early on, but then ran out of time to properly flesh out. Fingers crossed they'll do something better with it in PoE2.