Good observations.
To decode that, are you saying that the only real obstacle is getting a Velbus connection between your loft space (and upper floor lighting etc) to your downstairs system?
If you have a LAN cable between the two points, that would be best or you can use a WiFi version of the following…
@jeroends created an amazing Velbus to TCP server, which works incredibly well.
https://forumtest.velbus.eu/t/how-to-install-and-run-velserv-a-velbus-tcp-gateway/15422
But what isn’t know well is that it can also be put into Client mode and bridge Velbus networks.
In essence, you’d create an isolated Velbus network in your loft space with it’s own VMBRSUSB, when you’re happy that it’s all working, connect a very simple Debian based mini computer with nothing more than VelServ in client mode, connecting to your main system over LAN.
(This assumes you have a bigger / another mini computer on your main system that is running VelServ or Velbus-tcp, as a network Velbus server)
Once linked, your whole house will appear as one big Velbus network.
This same concept can be used to link between the remote buildings.
https://forumtest.velbus.eu/t/wireless-data-transfer/15666/3