Regards the multi processing via fx in ableton or any daw this is only possible if the synth has multi outs along with the interface of the daw/PC having multi ins and you can route the synths sounds to its own outs, not aware of that many synths that do this, the analog keys does but unsure if too many others.
Using a handful of multi mode synths with the OT right now, a blofeld, a Nord, ms2k and a microkorg XL, I have also used Halion and several versions of Monark with great success so it’s not always hard synths. All seem to work pretty well but you need to be on top of your midi pages so a memory or an iPad note/paper pen whatever are good advice, my fave cc are dependant on the synth to be fair, the Korg for example has the ability to scan it’s waves from cc ( or internal lfo) which when modulated is pretty cool especially when tied to heavy delay and reverb as the sounds that are created can be very cool and you’d never guess where thy came from, you can be very precise of how many waves you desire or totally random and I think there’s about 70 in there, I’m not going to offer a mini review of this under rated synth that many call a toy here so I’ll shut up.
I’d say I immediately go to filter cutoff, drive and also osc shapes and mods as my first cc to map, if the synth has clever envelopes I place the cc in for those too, such as a looping envelope so the decay turns the envelope to a separate and adjustable lfo if you like. Obviously every synth has its own quirks that you will enjoy so they will end up being a choice.
Honestly, integrating synths with the OT is so easy that I encourage it whole heartedly. Multi mode synths are brilliant so long as you keep control of the sounds, I do not process them to audio until I need to, if I am staying working at home then I just store the midi settings in the OT and turn in the synths with the OT projects. For those that are becoming part of the work I take away and if I cannot carry the extra synth then they obviously get mixed down and will become static tracks.
Assigning cc to a scene is as easy as anything else I expect, I’ve not done it but I might just do one now to see if it’s how I think.
Edit: ok I’ve never tried scenes and midi like I say, quick effort says that the fader doesn’t work on cc as with audio (that’s a real shame now I’ve tried it I’m intrigued with ideas) but I expect that could be worked around with clever manipulation of midi and a translator of sorts.
Dive in I say