Two things.
First, omnimovers are ever moving ride vehicles. Any time something gets in the way of that, like a preshow, it can’t eternally load. The Stretching Room stops the queue instead of letting guests continually board. You’re also right about the FP as evident by Astro Blasters, but FP to Mansion is new and it’s always been backed up.
Second, I have a memory as a kid where one of the elevators was broken and there were boxes blocking the door. Despite that memory not making a lot of sense, there’s no other way I, back in kindergarten, could’ve known there were two elevators unless there was something out of place.