MC does not use a "strict" naming convention. It tries to parse
many common naming conventions:
http://wiki.jriver.com/index.php/CarnacThe filenaming structure you described should work fine most of the time (as you indicated, as long as there are no movie name collisions). Metadata in existing JRSidecar XML files will "overrule" data MC pulls from Carnac. If the information in the sidecar files is correct, there's no reason to delete them, as they'll be used.
MC should still do the metadata lookup and get cover art on import, though it won't fill anything explicitly listed in the sidecar file (it won't override a sidecar file automatically). But otherwise, it still does it. If any get through with missing data, or stuff from the sidecar is wrong, you can always select them and do the lookup manually. More info on that system is here:
http://yabb.jriver.com/interact/index.php?topic=69316.0