Chris,
Just make sure you are happy with whatever outcome is agreed. It sounds like you're a rational bloke so keeping calm and exploring all possible solutions with VWI for a solution is the correct way to go about things.
BUT!!
If it were me, I'd not be happy with a retro fit because of the probable removal of interior etc. I'd just want the product to be as I expected. What were they thinking knowing that 4 GTD's we're let out into the wild missing standard spec!!!
Also, the fact that VWI offer Irish customers such a miserable low spec anyway to have the electric mirrors removed also is just taking the p##s!!
I hope it all works out for you and will keep an eye out for you on my travels!