Thank you, both, for your further input on this.
A few further interesting developments:
According to the Kodi forum:
https://forum.kodi.tv/showthread.php?tid=339620
It does seem that the issue that I was dealing with has been raised as a bug. With a bit of luck, if they introduce folder watched statuses back into file mode, this would pretty much solve my problem.
I also spent the week messing around with Emby.
Initially, things went particularly well - it did seem to solve my issues. Files were added to the TV and Movie library without needing to be scraped (which was exactly what I was after), but then I started to get a few issues with the Emby server suddenly not being available.
I ended up uninstalling it again. It is a great piece of software but it did seem massively over-complicated/bloated for my setup and what I want to achieve.
I also did some testing with my Metadata idea above and, as you predicted Em, it did not quite work. None of the important additional information was included in the .mkv file, so I was no better off than having recorded it as a .ts file.
I wonder whether all I can do is just wait for the fix in Kodi - I seem to have exhausted all the reasonable options.