I'm not sure how one would be able to play a "note off" as a "mono" or "poly" as those are just single hits while the "note off" has a user controled length/decay. Maybe the solution might be to use the same sound on separate pads but both with their own poly setting. From what I gather you want to be interchanging "note off" with "mono" correct? record the "mono" pad on a separate track and use the "note off" on a separate track to touch it up.
If that doesn't get the desired effect try to get more exact "note off" information input into the sequence. You can do this not only using the pads but also with step edit (as I mentioned before). That way you'll get those precise input points without having to play it perfect or with quantisize on. Also the changing of the temop does not affect the notes played on a "note off" pad as they will stay on cue within the time signature they are assigned to.
What is very troubling about the whole "note off" function is that you can never play more than one sound from one pad at the same time while recording on a track (say doing a tune in 16 note mode). But you can record that data many times over in a separate track and never have the notes interfear with eachother. Because of this handycap you can't always play your mellody as is but on the plus side it does makes you think more about it's construction in more detail (that or just copy the pad over a couple of times at different pitches)
