Larian has already expanded the UI so that you have room for six character portraits. It would't be dificult to make a six man party available for the players. Those that want a four man party need not take any more, but we who want a party of six would be free to do so.
It certainly wouldnt ... But its even easier to simply say "we are counting with that many people will use 6member party mod, so we counted with it when we created our UI ... but that is all support you get from us" ... wich is basicaly what they did.
Any problems related to that mod will then follow the mod and not them.
That is a valid option for Larian, sure. But it doesn't mean much if the game still doesn't reasonably support 6 players; e.g., the auto-removal of players 5 and 6 on the raft scene.
Lack of formations is something Larian *could* leave as an issue because it doesn't break the game, it's just not liked by some people. And it's an issue not specific to 6-players; we can't change formations with 4 players either.