How badly things could have gone if the diamonds had tried to unfuse and poof Stevonnie as they did the other fusions? It feels a bit strange that Yellow and Blue did not force or even command Stevonnie to unfuse as they did with the other fusions.

I have two thoughts. One, holy cow was Connie close to possibly straight up dying there. If we remember Steven's half-immunity to the gem dematerializer thingy from previous episodes, it seems logical that Connie will have full immunity. If the diamonds had attempted to force Steven and Connie to unfuse and found their materializer unsuccessful... bad things could have happened.

Next, I think there are two reasons for why we didn't see Stevonnie coerced into unfusing there. I think that firstly Blue and Yellow didn't want to make any more of a scene than had already happened. I would assume that their leadership is held in place almost entirely by their reputation of being virtuous and infallible leaders.

The second and more practical reason is that I think the writers are going to recycle the "humans are immune to that specific gem weapon" to give Steven and/or Connie a surprise that will be important in a later fight scene.