I would ring up one of the dealers and ask to speak to the motability specialist. Mention to him/her that on the auto trader they have the Audi you want and can they check if that particular spec is available on the motability scheme? They can check their list to see if that car can be released to a motability customer and they will tell you what the AP is. Do note that if it has extras fitted you might have to pay for them.
You could ring them on Sunday to see if those cars are available for you before ringing motability on Monday about existing order. If motability says no the car you ordered can’t be driven by under 25s then you can mention that there is a dealer who has one in stock and that you are going to order it. See what they say.
Just realised one problem. Looking at the motability website today there is no under 25s Audi A3s. That model you ordered is no longer on the list. Best to ring motability on Monday and if you get a refusal on your current order ask them can you order the one that the dealers have that is for under 25s but no longer on the motability scheme?
Look forward to any updates.