not sure if it the following is not possible to do, or if I am only unable to figure out how:
It's possible indeed
- editing the velocity of recorded notes in the sequencer without re-recording (either individually, or globally)
(Edit: I'm not sure from your question if you know the step edit method that allow this - I'm describing it anyway, just in case, and for the benefit of others)
This can be done by using the step edit mode, and then simply re-recording the note. To enter step editing, hold the matrix button for the step you want to edit for a little while. All other steps will then turn blue, and in the top or bottom of the matrix (depending on if the the chosen step is in the top or bottom half of the matrix) will show a blue button (or more blue buttons if the step has a chord recorded). Now you can hit a key which will replace what was in that step. You can hit the key repeatedly while the sequencer is playing until you are satisfied with the velocity.
- somewhat related: can the sequencer be set to a fixed morph, e.g. similar to "real" split mode where the lower part is always Morph A and the upper part is variable?
The default mode of the sequencer is that it morphs, but you can force the sequencer to play the A sound, and then you can play in the upper part with A/B morphing. When the sequencer is first engaged, you'll see in the Timbrality section that the Single LED is lit and the Split LED is blinking. If you press Timbrality twice, advancing the mode, you'll see that the Split LED is now constantly lit - you now have the "real" split mode, as you say.