finally, i finished off by trimming the lhsf eyebrow to the face using boolean operations.
I couldn't seem to get them to work how I wanted, to trim the eyebrow bit. And selection order didn't seem to change the precedence in the boolean operation.
In the end I ended up doing a copy and paste replacing the face, then a copy of the eyebrow with no paste, then the boolean operation between the eyebrow and one face copy, then paste from the eyebrow copy the style (opacity/blur didn't seem to copy, just the colour, not sure it stroke styles copied or not), then deleted the original eyebrow. And renamed everything correctly again using the XML editor.
I noticed you can't do a multiple select in the XML editor, only select a single element.
So, I think I'll raise three bugs from this, boolean operation precedence, style paste not pasting blur and transparency and maybe stroke styles etc.. and inability to select multiple shapes/groups via the XML editor dialogue. I'll leave the snapping as an enhancement to look into and not raise as a bug as the enhancement I suggested may be best as a different snap modulator.