somewhat random questions - but in Cubase, you can have a rack of instruments, and then using Halion as an example, fill each slot with a different sound - each one with a different midi channel.
If you create a midi track, setting it's output to the right destination, as in Halion, and sending on channel 2 for example makes the channel 2 sound work, another track can send to a different sound. The main drawback is that the mixed output of the Halion in stereo is what feeds the busses, so you can't eq each one separately, or send certain things to processors etc. In the midi track in the mixer - you only have access to MIDI processing, so no reverbs delays or other useful stuff. The alternative is to use instrument tracks - each one is then treated as audio - you have the same VSTi, but a separate one for each track. Years ago this was a processor bottleneck, but now it seems to make no difference. Each one can then have it's own reverb, processing, eq and any treatment you fancy.
I use instrument tracks as a matter of course now - but old projects I open have rack instruments and I usually swap them all for individual instances of the synth and samplers.
My default output is a single stereo bus - not sure why you'd want just the left?
Are you using the Steinberg as the MIDI in and out? What exactly is hanging of the MIDI output? I thought your synths and stuff were all VSTis? The remote control section is different from the 'music' midi outs - can yo0u replay MIDI and make something outside happen = if so, the outputs are active. In the inspector, what are the MIDI outputs showing? Presumably your keyboard is getting in and being routed somehow? I'm not quite sure what you are doing?