Is it possible to link the nextcloud box to the TV using USB and mount a specific folder from the box so the TV sees it as an external drive ?
I wouldn’t say so “out of the box” unless you can effectively reverse the use of a USB port on the Pi to serve content?
You’re right, looks like you can only way to access the Pi is using a TTL serial cable, not sure it will be really effective to stream movie.
I just remember that my TV have an ethernet port; might be easier to share this way (DLNA or whatever)
Streaming would likely be the easiest option. You could use a USB to ethernet adapter for a second NIC, put a crossover cable between that and the TV and use it as a dedicated transfer method for streaming.
Or, you know, DLNA or whatever 
I’ll get a ncbox when it will be available on Pi3, Might be a way to initiate the stream from the Files App or a specific Media Player App