It seems that you are almost looking for reasons for things not to work.
If you have a Garmin head unit then you are highly likely to be registered with Garmin Connect (you won’t get firmware upgrades otherwise) and you can then just link GC with TR.
I just move the head unit between bikes, just need a mount for each of them. I don’t have a power meter on every bike.
The sync just happens: finish ride, save the file on the Garmin, the Garmin auto uploads to Connect, Connect sends it to TR. By the time I’ve opened the front door it’s been done. I just have to refresh the calendar and complete the post workout survey.
I’ve done outdoor rides, usually Z1/2 style, without following a TR workout file. When I’ve got back I’ve been able to associate the ride with a TR workout. If there was a workout scheduled for that day then that is the default option.
Your request about telling TR/AT that you have completed a workout without providing any data for it to work on doesn’t make sense, you wouldn’t do that with a real life coach. AT is all about data, comparing expected and actual values, how do you expect it to work without that data?