I see you have released your mod. A note for future endeavours: as a general rule you never want to touch the bottom row of neck verts on a head, as they are positioned so that they properly mate with bodies that don't have a hidden join. If you give that head a different body, like clothes or a robe, you'll see a massive gap at the front now because of the change you made to the position of that front vert. It's not a problem in this case, but something to bear in mind for player heads especially.
As I said above, this looks like an issue with the body model, with the way the collar verts on the front part come in too far, Presumably this is also a problem with other heads. It may warrant a more general fix.
Regarding the clipping at the back, that is presumably because the collar on the body is weighted entirely to torsoupr_g, while that area of the neck on the head has a reasonable amount of influence from neck_g. This is again something that could possibly be a more general problem, or maybe it only affects Twi'lek female heads, since they were never intended to be used on other bodies. The solution would likely again be to edit the body model, this time adjusting the skin weights of the collar. This may necessitate adding bones to the rig, as body models typically stop at torsoupr_g and don't have the neck_g bone. Alternatively, the head could be edited to set the weights on that area of the neck to necklwr_g (which is functionally identical to torsoupr_g). Either way should stop the clipping.
Edit: Here's Mission demonstrating that the problem is at least partially with the amour models: