Firstly

and thanks for sharing.
Secondly What can we learn from your experience
Well, reassure yourself that your diagnosis of almost asleep was probably correct (although totally sozzled or drugged up are other possibilities). What could you have done to help him? Probably nothing - unless a good scare would drive him into taking a power nap.
What could you do differently in a rerun? If you come across one whose performing erratic or exaggerated movements such as the first pothole you witnessed him miss, unless there is compelling reason to do so, discretion may be the better part of valour and it may be worth sacrificing 10 mph for a while to appraise him for longer. Having decided it is sleepiness, the best way to overtake him is to hoot a couple of seconds or so *before* pulling out. Hopefully it will bring about an almost Pavlovian chink to the left, whereupon you can safely and swiftly move past. If not - and it may not - be prepared to abort that overtake.
Just my 2d.