Once a game is started, any additional people joining would only be able to take over a companion (if available) or henchman. Henchmen do not have personal quests, and there are restrictions on what recruited companion characters can do compared to an avatar character, created in character creation (specifically, origin moments and dialogues for companion characters are only triggered if there is is an avatar character in the same party, controlled by the same player).
If you hire a henchmen, your friend would be able to respec the character on the Lady Vengeance. The level and type of starting equipment would depend on the henchmen (should match the character doing the hiring IIRC).
Yes, you can start a new game and have everyone join in character creation, and that would indeed require at least 2 systems (one using split screen with 2 controllers, and the other joining online).
P.S. the restriction have been removed on your account, so you can use UBBCode now, if you wish.