There is no “grey area”
If you sign a new vehicle sales order with a dealership you have committed to a contact for them to supply the vehicle.
You are correct that the contact does not start with Motability until the PIN is entered, this is separate as this only guarantees the purchase side of the where the dealer will be paid, the exact same thing of you where taking any finance.
For the sale of goods the actual contact is made when the seller (dealer) accepts the money (deposit) from the customer this forms the sale and the signed contract apply.
If the contact you signed states no deposit refunds or that you are made aware of this verbally then here the terms are clear.
Should anyone cancel a vehicle order where they have paid a deposit it is entirely unto the discretion of that individual dealer to refund the customer should the wish to.