I’m in the same position as you and seat comfort and hight is crucial I found the same problem with the Bmw active grand tourer, on model we tested I couldn’t get comfortable at all.
The dealer told us the seats were changing to a more comfortable seat but there was nothing like it in stock.
so we left it.
if the car is causing you physical pain then it’s not the car for you, Motability can and should help you, It may be they suggest a small cushion at the base of your back and then see if the seat softens up?
But I would suggest they let you test models with different seats, maybe the seat with lumbar set to off is what you need, rather then a standard seat which must have a small amount of lumber support in built.
as you say a short test drive and a dealer telling you the car you order will have suitable seats is asking for trouble by bmw as how would they know until you tried them for yourself.
I hope you get a suitable outcome.