On this version, the mirror kind of slots into the metal plate on the windscreen, but basically wobbles all over the place without the bolt. The bolt goes through the base of the mirror which is threaded with the end pushing against the plate. Basically it is the only thing that really fixes it to the screen - it is pretty intrinsic to the fixing of the mirror.
Glue might be possible, but it isn't a direct fitting between the two sections as they have to be slid together - it would probably cause a bit of a mess and there might not be much glue left in contact.
It seems pretty odd that they wouldn't have just gone for a standard metric bolt.
I keep trying sellers who are breaking or those selling the mirrors themselves on eBay, but have had little joy in finding someone who hasn't discarded the bolt. I have one option at the moment which looks like the right thing but a long delivery time for some reason. From a 2010 model so I hope that wouldn't have changed...