1) If you're just going for maximum damage, then no, there's no reason to not go for dual-wielding. But if you want to try different styles and weapons, feel free to choose something else.
2) Use whatever you like. Strength gives bonuses to health regeneration, Intelligence to mana regeneration. Over time, that adds up, and your Strength bonus can give you such a boost that it makes you much harder to kill. But that could take a lot of the challenge out of it. I'm planning to make an intelligence-focused warrior just to make things more challenging.
3) Possibly, it depends on what you use your mana for. Warrior skills tend to be light on the mana. Rangers are the ones that have the most mana troubles, I find.
Life Tap is a really great skill for warriors, though, especially Strength-focused ones, because Life Tap drains health, and strength-warriors have higher regen, so they get that back. Intelligence-based classes have little need for Life-Tap because of the inherent high mana-regeneration from Intelligence.
4) Yes, but it's better for ranged classes. It has a high health pool, and it charges enemies and fights them in melee, so it makes a great distraction. It's not great at killing things, but no summon is. If you're having trouble with groups of enemies, the demon is a great help.
5) I've never really used it, but my view is that no, it's not worth it. Enemies in the game tend to do high amounts of damage, and extra armour isn't that helpful when you could be using Battle Rage to kill them faster.
The upkeep for the damage-boosting spells is worth the trouble. They tend to last a long time, except for Way of the Ranger, but that offers a higher boost.
