1. The witty answer is "git gud", but really there is few reasons why you shouldn't play with one custom character if you want to. You're not missing out on much, so if you feel DOP is too valuable then just go for custom again. Two playthroughs with 3 origin characters is all the origin characters anyway.
2. Lohse is an interesting character as a companion, and probably is as a main character too. The word on the street is that Fane lets you join the Voidwoken but I don't think he's a prerequisite as you're supposed to do this via the Sallow Man. But ultimately I don't know.
3. Having played with Beast in playthrough 2 I can say the Act 1 and Act 2 quests related to Beast do barely change. I haven't seen Arx yet but I suspect unless you like Beast as a character there's no great incentive to. That said, Beast is fun and has a strong special ability so I can't say I didn't enjoy it.
