I usually don't have any issues with silly science in Trek, but the idea of high atmospheric pressure affecting incorporeal energy-thought beings who are remnants of fleshy folk (aka ghosts) was a bridge too far for me. It's just too inconsistent with the premise of the Lights.
Amen to that. But couldn't the point be that the pressure was a way to torture the g... Mira Romaine without actually doing serious permanent harm to her, thereby making the ghosts very uncomfortable? I mean, it's not as if they'd worry about Romaine feeling pain as such - but they'd be very
uneasy with the prospect of her body failing. They even express moral qualms about the earlier deaths, adding (ever-so-slightly) to their motivation not to remain in a dying body.
Sure, that's not what the writers meant, but it's better than trying to believe high pressure is the "environment deadly to the alien form" they speak of earlier on... And it's consistent with Kirk's threats about putting an end to it after speaking about cost. The Lights would
have every reason to think Kirk is killing the g... the female guest star.
Placing Romaine in zero gee would then make it doubly enticing for the Zetarians to leave the body of the... woman and return to an environment they are more comfortable with.