Ok I've been thinking this through. Neither RTWP nor Turn based make sense for a multiplayer game
Turn based is boring if one of the player takes too long (I am looking at you Scythe ) RTWP is essentialy RT since nobody gets to pause the game.
Now, let's take a look at how some japanese RPG do it : intiative gauge.
in multi, each player could choose an action during a turn that last 6 seconds then all actions would be resolved according to initiative. in solo play, it would be essentialy turn based.
I'll be honest, and I know this doesn't really matter, but I could not care less about multiplayer, especially in a game like this.
And I really hope that in terms of design, Larian doesn't prioritize mp in the slightest until well after they complete the bulk of the game's structure, not the least of which being... ahem, the combat system.