WHY would it have been necessary for Carla to go through negative space/time?

[anagogy]

I wonder how rare it is among wanderers who are capable of falling into a trance state. That qualification probably drops the number of possible wanderers to a small fraction of the total wanderers. No wanderer is going to get that opportunity to choose negative if he's living this life unaware of who he is, right?? and most wanderers are unaware, i think that's what Ra said.

There is always a choice to become negative, whether in trance, or not. But the circumstance of being led away into negative time/space is extremely unlikely. It is slightly more likely (though not significantly so) to leave your body for the purpose of "service to others". Essentially, you try to do a great service, you will inevitably be approached by dark beings who don't like you doing that.

But again, its like walking outside during a thunderstorm, getting struck by lightening, in likelihood terms (even less so). It is a function of innocent gullibility to follow negative beings, and then gradually listen and accept their negative philosophy (though some are no doubt extremely good at persuasion i.e. think of devil archetypes, Faustian deals etc.).

you said "gradually lead them." but Carla's experience, from start to finish, was a few minutes, less. so does that mean that Carla, in the interaction with the negative entity, would have been in an essentially timeless state? so really it wasn't "a few minutes' worth" of mistaken choice? it would be a timeless choice.

Exactly. A few minutes of space/time is infinite time in the realms of time/space. It wasn't an "instantaneous deal" no matter how it looks from the space/time vantage point.