As others have said, you could ring the dealer and request a quick handover and set the car up, pair phone etc at home yourself. You could also ask the dealer if they are willing to email paperwork for your fiancee to sign and allow you collect and enter the PIN discreetly or you could just ring Motability and explain your situation and ask if they can help in any way or authorise tou to collect it, wih an appointee disclaimer.
However, most likely your going to be stuck between a rock and place and will have to endure the procedure. Maybe, sending your fiancee to do most of the paperwork and enter the pin, whils’t you look after child and swop over, whils’t you take delivery of the car and sign the driver decleration may help.