[schapplm]

I am looking to add a bunch of manually created nfos for my home videos. I would like WatchedList to take these into account. Since (if I understand correctly) WatchedList uses the IDs which are stored in the NFOs (I am assuming this is so - I use local scraper only), can I just makeup any ID/UniqueID and have WatchedList do its magic?
...
Are there any restrictions on this?
If this doesn't work, is there a way to pull this off?

WatchedList uses the IDs that are in the Kodi Video Library itself.
The corresponding columns are "imdbnumber" for movies and "tvshowid", "season", "episode" for tv episodes. I use the standard-json-queries VideoLibrary.GetMovies and VideoLibrary.GetEpisodes.
So if you somehow get Kodi to recognize your home videos as tv episodes, then it should work.
I have no experience with the uniqueid-tag though. I think it will only work with tvshowid-season-episode convention.
IMO there should be no restriction on the IDs except perhaps being integers. Your numbers are above 32 Bit integer scale (but below 64 Bit). I do not know, if Kodi python uses 32 Bit or 64 Bit Integer precision. To be save, you can choose smaller values.