Hi Stephane!
In answer to your questions:
- Controlling intervals in counterpoint is certainly possible; just depends how you want to create the counterpoint.
- Sending key switches via MIDI is possible, though it requires a little bit of wrapper code. (See this thread: MIDI instrument for typical orchestral samplers, keyswitches, etc - #3 by MikeMos)
- Dynamics are possible, at least in terms of notation. You have to change the playback velocities manually.
- Live coding: people have experimented with it, and it kind of works! But it’s not really by design.
Welcome!