I can answer a few of these:
Playing an origin as your main character is essentially another nice story arc. I'd suggest doing it rather than rolling a custom character: Of course you can follow the story arc of the companions you invite into your party as well and see the origin story that way, but I personally think the richer the story of 'my' character in a RPG, the more interesting and fun it is.
Your companions always auto-level to your xp when you invite them back into the party. That said, I'd suggest picking a fixed set of three companions for your playthrough and not to cycle them. The reasons are twofold: a- you can easily miss important location- or encounter-triggered dialogues that may be essential to furthering that companion's questline (read: that questline will be irrevocably stuck if the relevant companion happens to not be in your party at that specific time) and b- that way you have new stuff to see for a 2nd playthrough, once you do find the time to do one.
Most aspects of your companions (attributes, skills, etc.) can be changed when you recruit them, and also again (at will, by the way) once you have access to a certain location (spoilerspoiler).
