If you're generally working with ensembles, then the phasing effect is less pronounced than with exposed solo lines. You might have decent luck playing an ensemble patch crossfaded and then adding a solo player on top and using a slightly different curve. Generally though when the whole orchestra is playing, the phasing effect is really not noticeable in anything but the most exposed instruments. Some instruments hold up really well to velocity crossfade too, like flutes. I've played around witih a lot of the ensemble patches too and compared the velocity crossfade version to the dynamic recording and you really can't tell them apart.
I've found MIR is brilliant at hiding weak crossfades too!
Anyway your results might be even better after upgrading to Standard instead of SE because certain patches have more velocity layers in Standard. Legatos still only have two, but the legato patches are sampled in half-tones in the Standard and Extended libraries and many other patches get extra velocity layers.
As for what's going on "under the hood", I couldn't tell you, but it doesn't sound to me like the velocities actually start to mix until you get closer to 68. It sounds, to my ears, like you go from somewhere around 1-30 or 40-ish with just volume being affected, then the second layer begins to get noticeable right in the middle, and then you have just the top layer audible after a certain point.