It is a poor assumption that the TKD person you're working with has a lot of experience dealing with hand techniques to the head region. If he was trained in Olympic TKD sparring only, the only thing that would be going for your head would be your opponent's foot, which is usually pretty easy to see. The defense for that is either evade the kick or raise one of your hands/arms above your head to protect that side of your head. It is a gross motion and not usually drilled. Thus, it is not necessary to keep the hands up to protect the head area. You can watch what Olympic TKD matches look like on YouTube.
To be clear, Olympic/Kukkiwon/WTF TKD sparring is purely sport and definitely not combat preparation. Yes, it contains some elements that would be useful in an SD situation, but it is not SD or combat at all.
Also, pretty much anyone can get a 1st Dan in TKD if you show up to class 2-3 times a week for 3-4 years (in Korea make that 5 times a week for 1 year!). 1st Dan (even through 3rd Dan) is considered a pretty low grade in TKD. 1st Dan is considered an achievable level for pretty much anyone who wants to participate in the sport.
I think what I'm saying is that your expectations of what a 1st Dan in TKD should be able to do may be a little off. I would not expect that just because someone holds a 1st Dan in TKD that they would even survive an encounter with multiple assailants. As others have mentioned earlier, they hand out 1st Dan to 76 year old women! Yes, the 76 year old woman has achieved something in the art, but is that woman a deadly weapon ready for combat against any number of assailants: no. On the other hand, I've seen 18 year old 1st Dans that were absolute weapons in street fights; definitely people that you'd want with you when the going got tough.
You have to look at the individual, not the belt to evaluate someone. Not all TKD practitioners are the same. :wink: