I'm not sure this is all correct.
MC will automatically create Sidecar files whenever a video asset is tagged, as long as Sidecars are enabled. You can automatically create sidecars on Import (which I do for all of my video files) by simply tagging them during Import, if you enable the option under Tasks in Auto-Import's configuration to Write file tags.
If you want to make sure that everything gets a sidecar, just set up a very simple Tag on Import rule that tags some value to all the files. I actually have some setups where I made an integer field called CreateSidecar, which I fill with a 1 via a Tag On Import rule, and that ensures that every single file gets a sidecar created. But, if you add any valuable tags of any kind, the sidecars will get written.
I'm not sure under what circumstances MC wouldn't read sidecars on import. It definitely does for me. Are you sure you're not testing with files previously imported into MC? If so, you have to make sure to remove them from the "hidden" Removed Items database or else MC does NOT re-import them (and rather undeletes them, which means they won't get imported via the normal mechanism).