According to the moderator, Achroma, the 10% added to arena was removed and it is only initiative now. So no need to worry about extra stats. As for exactly how the initiative is added into arena I'm not sure. I think it's according to your move order (just from speculation). Like move 1 initiative goes to move 1 in arena and so forth for the other moves.

I think the pairing more has to do with who is doing arena at the time, but I really don't know.