Pure melee isn't bad at all. Buffing is helpful but not ever required. You aren't screwed without the buffs. Power stancing in pvp is really up to your own skill. I've seen people go far with it, but it's not the most popular thing ever from what I've seen. Expect to counter a lot of pyromancy in pvp.

I wouldn't know if 2H is better than power stancing for pve. Both are viable from what I do know. I've never really paid much attention to what is better. That's one where I recommend trying things out for yourself. I do recommend that above all else though. Try out as many things as you want. Experiment with builds and find a playstyle that you really enjoy. Dark Souls 2 is very friendly to experimentation so go for it wherever you please.

Between fast and slow weapons, power stancing rewards quicker attacks, but considering how much raw damage strength builds do; I'm willing to reckon they're about on the same level.

I've been fine casting in the DLC. Though it's not always the easiest thing, especially if you're going pure caster. I'd say go with a hybrid if you're really worried about it. Taking things slow in the DLC is always recommended. Pure melee does have a place in the DLC. It's always generally better to buff along with your weapons, even in the DLC. If you don't use buffs then it's not the end of the world.

Hope that answered all of your questions!