Head

If I need to signal-in a midi note from the piano roll in live, what port do I set as an input for the midi- or notein object?

midi-ports-in-ableton

https://cycling74.com/docs/max6/dynamic/c74_docs.html#live_limitations

even if in edit mode you can see multiple port available, when you use your device, there is only one port, and this is the one "plugged" to the Live's track of the device.

I suggest you to read https://cycling74.com/docs/max6/dynamic/c74_docs.html#live_limitations especially MIDI limitation.
even if in edit mode you can see multiple port available, when you use your device, there is only one port, and this is the one "plugged" to the Live's track of the device.


MIDI PORTS IN ABLETON