So if JRsidecare files exist, which takes precident, the .xml files or the information stored in the library? Are there any internal functions to stop these two sources from containing conflicting information?
The Library. There is no single/hard poka-yoke to prevent conflicts; however, the default Options should keep them in sync. Specifically, the ones I mentioned earlier. See:
http://wiki.jriver.com/index.php/Sync_the_database_and_tagsWhen you say that a file supports tags, does this mean that the information is somehow embedded in the file itself? How can i tell if a video supports this?
Yes, embedded. It may help if you familiarized yourself with the distinction between Fields in MC and Tags:
http://wiki.jriver.com/index.php/Tags. MC allows an almost limitless amount of customization regarding what metadata you would like associated with your Files by using Field. While the terms are often used interchangeably, not all of these Fields, especially User created Custom Fields, are supported by different Tag standards.
There is no way, that I know of, within MC to tell which Field is supported as a Tag by what File Type. Unfortunately, there is not one Tag standard that rules across all File Types. You would have to look up what Tags your File Type supports. This is true for all Media File Types, not just Video.
Please start a new thread regarding your Thumbnail issue to avoid thread-drift. It will be easier to help troubleshoot if you describe in detail the steps you are taking and what is happening, rather than just "it fails no matter what I do." A screen-shot often goes a long way.