Hi.
The jams are not the problem. As EricMoinet said, you can drive a track without jams: you will not see the graphics, but the track works.
I did practice, qual and races on Michigan, never had problems. Never tested starting from the last position of the grid, i'll test it.
@evroic: about the number of jam files in the folder: i usually modify the track graphic, so that maybe some jam is no more useful for my version and i delete them. Sometimes i found tracks using jam files which where not present in the folder, and i added them.
You can use a track in whatever level you like, the problem is related to the MD file: so if the MD is programmed to be used in ace level and you race in other levels, the performance of the CCs will not be realisti. But the track works.
Many times custom tracks are conversions of Gp2 custom tracks, and have some issues: Michigan had a problem with a "bump" in one of the corner (i don't remember which) that always made the car uncontrollable.
And sometimes yes, i have found the same command repeted twice or three times, and the same for the jam files: but this kind of issue usually create a graphic bug.
And yes, expecially on custom tracks carshapes can create a lot of problems: usually a good old CTD (for example: Monaco, Adelaide, sometimes Montreal custom tracks).
Bye