Contact either BMW or Getrag and get them to comment. Don't expect any help from BMW though - as with insurance, they try to get out of paying for most things !!!
If you are 100% certain you have never recurringly 'crunched' the gears then i'd say there was some flaw in the mechanism responsible for enaging the gears.
As proof that the gear box is fine as a unit - there are 100's of modified cars out there with high bhp and torque, used on and off track that have no issues.
This is fact.
I'd say that they have totally no proof of your ability to change gears smoothly or otherwise.
Interesting to know how you fair against the usual customer facing teams we all enjoy !!! ...usually some gimp without any techy experience or understanding.
Good luck !
PS. If you were in the USA you could probably take them to court for character assasination !!!
ha ha.
Cheers,
K.