UltraHD Blu-Ray does indeed use 1080p subtitles for 2160p video.
2.) Who's stopping you from using VapourSynth?
3.) Most of the important plugins will have similar ones in VapourSynth. Usage will be different. There is a learning curve.
I think in VapourSynth you basically want to render the subs using e.g. core.sub.ImageFile() on an Alpha clip, then resize said clip to UHD, then blend it with the actual video. In the VapourSynth doc you can find a blending example for core.sub.TextFile().
1.) Did SupTitle() have an alpha mode? Don't really remember if/how. If so it should be similar. See
this thread for ideas on blending subtitles on AviSynth.