I think SH and Astarion unfortunately are the only characters for which the writers could somewhat explain them being "fine" (pretending to be fine) with poly when they needed one female and one male character for Halsin. Which automatically makes them candidates for being "fine" with the drow orgy and Mizora.
Karlach's the only one that wouldn't require a lot of rewriting to be okay with it. You could argue that because she's gone soon, she might just be fine with whatever, if it's more hinted at earlier.
However, they threw the idea out of the window in act 2 for all 6 companions. To add it back in again in act 3 makes no sense, least of all for the two that need the PC the most.
