Before Vampire started using the old fire exit (no idea when, but certainly wasn't the case when I worked there) wheelchair users couldn't get directly to the ride without going up the exit on foot. Good old 90s design, and even now still need to go down steps to get there.
Always been put on the back two rows of Wickerman, though only have experience through wheelchair user. The position of Vampire's queue has nothing to do with the decision for the back rows being used as this has been the case since 2007 at least (source; I worked on it). I presume Mandrill's strange system (I.e. they don't care where RAP users go) is down to that it's not got a traditional lift hill and any potential evac positions are low to the ground where this is not a high level consideration to them.
Reducing the amount of distance travelled AND having a consistent position for disabled visitors is essential to ensure evacuation situations occur sensibly. Without seeing the Wickerman COSWP I cannot say for certain if it looks at the wristband type for RAP users to determine if this is a general situation or not.
Though I'm sure some on here find it "cruel" they can never get back row on Nemesis due to RAP users.