I don't remember everything you tried, but one thing that's tripped me up is forgetting to set motion-blur=0 for the interior glass too. This can make you crazy since you think it's the material below it that's causing the trouble.
I've had some issues even with collision box present.
I'd turn on racer.ini fallback check in along and make sure nothing is missing.
I've had problems even on fully set up cars so can only assume its just buggy in some cases.
I run without it on most of the time, its still got some work till it looks as good as commercial game motion blur...
Racer looks fine without it to be honest, but it'd be nice to have it if it were good and didn't cost many fps