Sorry. I see the disconnect. I was thinking the game would cap you at a total of 4 customs to align with 4 player Multiplayer. And so, the standard for the game could be set to party of 6, and you can start with up to 4 customs - 1 MC and 3 mercs. Then you'd still have the ability to add 2 origins.
But, to your point, mercs would probably be treated differently than 4 player multiplayer. You could create and let go any number of them at will.
As far as dying. Hah! Who stays dead in BG3? - unless YOU kill them.