That’s awesome that at least part of it is working, I’ve never seen the lock pop up on digitakt before. Is it confirmed your controller has that +1 function? I’m sure if dude saw it in your documentation, then it must be there (I didn’t look to confirm at the time he said that), and it would make sense for a foot pedal to employ up and down as a +1.
Most hardware sequencers are going to send those messages as program change which will cause patterns to change on DT, but since it’s not a sequencer they must have made a simpler onboard solution.
CC120 isn’t listed in the Digitakt midi chart but as a general midi cc parameter I think it’s usually a channel mode message that mutes all sounding notes (as opposed to 123 “all notes off”). I can’t see that stopping or resetting the transport to zero on it’s own but if it’s working that’s awesome, maybe it’s combination of the 2 signals you’re sending it (transport stop plus mute sounding notes) at the same time?
Hope it all works out for you, let us know what you figure out.