If you are talking about the wedge shaped room, there seems to be more space if you hang back and don't enter first (or stay at the entrance and don't go into the pointy corner). My claustrophobia didn't kick in when I was in that room, luckily, but the shape of the room could be pretty claustrophobic in that one narrow section for sure.
To skip the preshow they'd have to save spots on the colored dots for you, otherwise there'd be no space in the ride vehicles. Or someone else would be left waiting for the next grouping, etc. I think that would ruin the point of the attraction for many, as you are put in groups to fit in the ride vehicles from the point that you exit the second 'preshow'. And thinking more about it, those 'preshow' segments are really part of the attraction - I don't want to ruin the surprise for anyone, but it follows a plotline and has lots of detail.