WOW you really are in a predicament.
Your concerns regarding the GCB are legitimate.
Your desire to stay on good terms with your neighbour + landlord are also legitimate.
With 2 or even 3 quotes your neighbour will of course want to pay the cheapest one.
If it was me i would explain to my neighbour that for the 3rd quote you will take the car to the Motability dealer where i got the car from and go with whatever price they quote sighting your concerns about the possibility of losing some/all of your GCB to your neighbour.
The remaining option will be to put the entire situation in the hands of the insurance companies which kinda removes you from any potential awkward situation with your neighbour/landlord.
Good Luck, i hope it all works out well.