Some parts and bosses in the DLC were somewhat confusing if you had to do them with other players or you could do them alone.
Then again, DSII in general is harder than many sections in Demon's and Dark have been. In Demon's there is a sense of balance throughout most of the game and in Dark, with it's stupid parts and everything, it was still a lovely game. But DSII (now that I did multiple playthroughs) I noticed that it makes you work for your success much more. The game retains a lot of what's fun about Souls but whatever Miyazaki did as a director in Demon's and Dark is cleary not in DSII.
Didn't they nerf Climax pretty heavily? I'm pretty sure they nerfed all Hexes, which is a bit silly considering how much effort you need to put into being able to use them with any decency. Much more than any other spell or weapon.
No idea. I didn't followed most of the updates DSII went through.
Balance in Souls though has always being all over the place. The games, in the PvP side of the things, are about experimentation over anything else. And everything that has to do with PvP works as an extension for PvE (which is were the games shine the most and are the most fun).