Keeping up with the theme of "almost completely ignoring the prac documents", the approach I took for the head was maverick, to say the least. I got about as far as setting up the planes for the different orthographic sketches then essentially tuned out. Instead of adding subdivisions down the entire length or depth then manipulating vertices along the new subdivision, I took a sculpting approach; starting with a solid block, using Cut Faces to delete chunks of the block that weren't meant to be there (from whatever perspective I was using), using Fill Hole then merging/connecting any vertices I needed to give the model the visual fidelity I was after.
I accidentally moved a few center 'join' vertices along on the wrong axis, but it turned out not to matter as once I'd finished creating the instance copy and getting the whole head looking correct, I converted the instance to an object (Modify > Convert > Instance to Object) and merged the corresponding vertices along the seam ('stitching' it together). When you merge two vertices, the position of the resultant single vertex is simply the halfway point between the two original ones, so any that had been moved along the wrong axis ended up cancelling each other out and being perfectly centered. When I realized this I gave myself a little hi-five as in the prac doc there's no mention of physically stitching the two halves together; had I not messed about on my own I probably would not have thought to do this.
Having said that, if I'd have followed the prac to the letter I also would have used a Subdiv object, and not a straight poly, but using Smoothing ended up making this pretty irrelevant in the end anyway.