The dealer could have reduced the AP and you were within your rights to cancel right up to the point of putting your PIN number in.
Then there’s nothing to stop you ordering another car without penalty, trouble is once you put the PIN number in you’ve excepted the car and the agreed AP is due.
lots of people find dealers are willing to reduce AP if there is a reduced price in the next quarter but it’s down to the individual dealer, but normally pointing out you may cancel is enough to sway things, it’s only changing the motability order number. No big deal and the dealers not loosing anything bu doing it.
Only you can decide, but you can still tell motability the car doesn’t suit you and you want to return it. You may have to pay, but plenty of others haven’t.
look around and decide a different car or reduced AP suits your needs and place an order.
Motability can only refuse if you’ve returned the max amount of vehicles in the past, so there for are no longer eligible for the scheme, but that is rare.
As for the poor mpg, it should improve with time as the engine loosens up, but in reality short journeys aren’t going to give the best mpg.
A hybrid or EV would be the most efficient got that.
Maybe call motability again and ask to have a case manager as your unhappy with circumstances and your current health, good luck.