> does SVPtube2 on Linux support Twitch.tv?
if it's supported in the latest youtube-dl then yes
> the VapourSynth-less mpv that does work on Mint (though maybe it only works when mpv is compiled with VapourSynth).
> SVP in VLC is also missing from SVP
> the mpv pre-compiled with VapourSynth doesn't even launch on Linux Mint 18
it sounds like you just don't have vapoursynth libraries installed
> is the SVP Control Panel supposed to be missing a close button?
nope