So it is, maybe you missed my initial post?
Nope, I read it all.
My point is that you are making lots of hard work for yourself, and ongoing maintenance. A good backup solution once setup is pretty much set and forget, and it can back up all versions of your movies, whether they are included in the MC Library or not. If you make a mistake with a file and need to recover the original, there it is in the backup. Discover you made the mistake last week, there it is in the Incremental backup from last week.
Having to do maintenance both on your media files for MC, and then compare to what is already on a backup drive, is doubling your work. Simplify.
Differences are mostly simple
My emphasis on "mostly", and that is the issue. You could create rules that support your simple example, but then you have an even bigger job when the situation isn't as per your example, and you wouldn't be expecting the difference. Even if you had said the example was always the case, I would be sceptical. Structured names of anything fail at some stage, as situations change. Then you get caught out by exceptions.
Basically, hard drive space is cheap, and disk sizes are up to 12TB each now. However, life is short and spending more time than necessary for maintenance isn't a good use of it.
Anyway, given what you asked, I had a bit of a think about it. You would probably have to base any solution on the file name you structure, as the attributes of the two videos are likely to be different. I'm thinking resoulution, audio format, compression type, etc. are all likely to change. There would be no obvious fingerprint for the files that matches.
So based just on name... sorting by name, the HD name includes the low definition version, or starts with it... but you aren't just looking for files that have a missing backup, you also want to identify files that have had changes done, or have been replaced...
I would like to compare the contents of my library against a backup drive, and see what file names have been changed, or files replaced (e.g. after re-encoding to other formats)...
What constitutes "changed"? That isn't reflected in the file name. A bookmark in a file will change its Date Modified. What is the rule for "changed"?
No, I don't know all of the parameters you want to consider, and even if it was only file name, I still can't think of a good solution.