Remember, this is the Divinity engine. This is most likely due to a restriction of the engine used and will not be able to be changed.
Bogus. Not only there’s no “engine restrictions” in place (how do people even come up with this stuff when it doesn’t make a lick of sense technically speaking?) but the game does support party of six characters already by save file editing.
And yes, the UI scales appropriately too.
And no, that’s not an ideal solution, which is why we are asking for proper official support.
I ended up with a bug after having Sovereign Glut in the party and raising two duergar with his spores. After they (the duergar) died in combat, I raised them with the scrolls of resurrection and they were added as party members. They showed up on the inventory screen and everything. Based on that, I'm really not sure there is a hardcoded party size limit.