I can't alter a watch face - i.e. alter the fields displayed. I'm not wanting to download a new face, just alter one already download and present in the app and on the watch, or alter any data fields around specific activities.

I get syncing to any 3rd party service is clearly totally out unless I manually transfer the .fit file via USB and I understand why this would need their servers, although it would be good if the mobile app allowed transfer of these files, without requiring a laptop and USB cable.

There's no reason for the entire mobile app to be totally and utterly broken because their servers are down. There's a huge amount of functionality in there that should just work locally without needing an internet connection. I can't even set an alarm via their app without it needing an internet connection!?