Larian has said you will be able to set the party size to six at game release. This is currently perfectly feasible for the game, but requires save game editing, so there is no reason for Larian not to deliver on this promise in the end.
There's some justifiable reasons they might not want to make this available during testing phases, though, mainly balance issues and similar.
I can't quite follow your logic here. Why would balancing be a reason to limit party size? In order to get balancing right they would actually need to unlock the maximum party size or else all the testing with an undersized party would make zero sense. Or do you think the new maximum of 6 characters is for later chapters only?