My brother recently bought an ex motability car and they completed and removed the bottom slip from the v5 and gave it to him before he could pickup the car as it was the salesmans day off, that my brother could collect on. As he needed to go to PO to tax it to change the class from disabled to PLG. So that should not be a huge issue at all. Maybe they don’t have the v5 yet themselves. seems a strange one?
Did you speak to the mobility sales person as sometimes they not available and the person who spoke to you may of knew nothing at all and just told you some waffle. Might be worth calling again or popping to the dealers if you can to see them or making sure it is them your speaking to, not just any salesperson at the dealers.
Failing that i would call mb up and explain the situation, they can contact the dealer and have done for me in the past.