Hey Cresshead,
Thanks for posting. Congrats for realy pushing yourself and attempting a most complicated subject thats definately the most exacting. You are pushing yourself way out of your comfort zone and as a result you are going to grow quickly as an artist and technician.
For portrait I would hold off as long as possible in regard to detail. Spend as long as possible on the basic planes of the face. It's a generic approach at first that establishes the foundations of the face. Check and double checking areas like the planes of the nose and make sure all the most basic forms are established.
When you have sweated over that by checking double checking and quadruple checking, then you can tweak the detailed nuances that make that characters the individual he is in regard topography.