Neil, I looked at your data, and I was able to snap to points of any elevation, not just the topmost.
I used 3dpolyline to connect the points below. Osnap set to Node.
Tim - I think your post prompted me to think OSNAP and I had OSNAPZ set to 1; when I changed it to 0 I was also able to use NODE to snap to locations at various depths.
Now to try and make some sense of the geology......