I feel foolish for asking this, but I can't seem to use or adjust a lip mask so it's up to the the edge of the lip zone but still soft edged enough to blend seamlessly with the adjoining face map. Surely this is possible. Help please.

I don't think it is. I've tried to do the same thing and always get that razor-sharp edge....your lip textures have to either be larger than the lips or smaller than the lips and have surrounding mouth flesh as part of the lip texture. You will also note that when you buy a character pack with multiple lip colors the vendors leave part of the mouth skin/texture to blend with the lips. It's just not practical to try to get right to the edge. Same reason I use iris textures  that are larger than the iris so I don't get that razor-sharp-robot-eys look on my figure.

W10, Ryzen 5 1600x, 16Gb,RTX2060Super+GTX980, PP11, 11.3.740