I'm with LVP...if the seat can move fore & aft...then the bolts that secure the seat rails to the floor can be undone.
If they are giving you a free loaner...and the service rep is trying to procure you a new motor (for free)...then you are ahead of the game.
Personally, at this point, I would have made haste to the dealership to see exactly what the issue was with my seat...along with explaining that the seat worked prior to them beginning work on the vehicle (especially if you really know that the seat was working in all directions before you took it in). And if the madness persisted...then I would have asked for a demonstration of the seat's movement...and if they only tried to raise the seat...I would have then asked..."may I"...then moved the seat fore/aft...and then asked at each full extention...are these the 2 bolts that undo this end of the seat...then smile...and say..."gentlemen, looks like we can move on with the recall repair...and I'll return the loaner when you get this all wrapped up".
If for some reason the up/down seat adjustment motor isn't working...by having them show you the motor...you may even be able to find a used one or buy from an aftermarket vendor and replace it yourself for less than $650.
The seat (rails) are held down by four bolts (see #4 in diagram below):
Item #5 in the diagram below is the vertical seat adjustment motor...and you can see that the one for the passenger side only costs approx $180 (BMW part # 52107111909 @ my local BMW dealership:
BMW Parts - Genuine BMW Parts and Accessories - Save Money ):