Yes, if you order and delivery of the new car goes beyond May, your current car will be extended until new car delivery day
When you go to pick up your new car, that dealership will deal with the old car. No requirement to return the car to the original dealer.
Ordering your New Enyaq, maybe someone who has recently taken delivery can help with wait times
Apparently, its been the case that if you order as soon as possible, if the AP changes, you can ask the dealer to change it to a lower price if required. Some do it no problem, a few say no. But given the changes in how the scheme works, motability dont like dealers doing this and may or may not change the rules. Either way if you are not happy you can change dealer to a more considerate one or cancel the order.
Others will add advice who have done this.
But the scheme is changing beyond all recognition in the next few months, and what was achievable before may not be in the future. Just go with current advice and enjoy the EV.