Hi. As far as I know, Spotify doesn't work (for selecting songs from within the FH app).
You can play any audio (music, audiobooks,YouTube, whatever) through the FHFX over bluetooth.
I've not tried the return of the fx loop for music. I'm not sure it would work as the FH will be sending part of your guitar signal off to the loop but it won't return so that might affect the guitar signal. I think it is something you could try but don't be surprised if it doesn't work out.
Whether you use your amp, home stereo or headphones is up to you. You should get better results from the home stereo/headphones, if you're playing along to a backing track through the FH. If you use the katana, use the 4 cable method and disable amp and cab sims on the FH.
As far as we know, there aren't any updates on the horizon so if you buy one, perhaps accept the unit for what it does now and if they provide more amps/effects later then that's cool. Just don't bank on it.