None of that is not going to happen at all.
A character's relationship with other characters is tied to the choices that character made. You cannot swap in characters around without it breaking. Higher level characters dropping in breaks the combat balance.
Others games have done it, including BG 1 and 2 and it does not break anything. And the relationships and choices there are as important if not much more.
People who do that know that the game will be easier.
I hope Larian chose to include it eventually, although it is not something that exists until now in their games.