I now know the answer…..
I created the basic route in Kurviger to include a trip up the road to the Casino restaurant. I then Airdropped the GPX file across to my iPhone. I then opened the file in BMW’s Connected app, where it displayed perfectly.
I also opened it in Garmin’s Drive app (it was perfect) and from there exported it into my XT, where it was perfect, too.
I guess the only useful thing about finding this out is if someone needs to send a precise route to someone, for example to take them as close as possible to the Casino restaurant, up a road that is not normally navigable, then they can maybe use Kurviger (which will allow the unnavigable road to be included) to create the route and then export it into whatever other app they like. It should work.