@FastBikerBoy Aha, so holding down Alt while changing velocities makes it only work on the selected notes! That's what I needed to know, that's terrific! It makes it so I don't even need to only show velocities for selected notes. Unfortunately, the rectangle-only lasso select still makes things difficult, but it's better than it was.
Usually the right hand is playing other notes besides just the melody, so I can't simply select the melody, I have to do a lot of Ctrl-clicking on individual notes. And isolating the melody on its own track means I would have to record in an unnatural way. But anyway, thanks, that helps a lot!