THANKS to all of you for the great and detailed feedback you have given!
It's a lot of input and it will definitively keep me busy for a while :)
@Rectro: Thank you very much for your kind offer to draw over my pic - at this moment I don't feel confident enough to subject it to one yet - Actually, I can immediately spot the points you have made in your post). But for my next approach, where I hopefully have incorporated all inputs, I'd be very happy to ask for a correction!
As for the ears, yes, I didn't model them - I try to stick to my 2 hour limit each day, and ears are not (yet) that important for general likeness. Also, as an exercise, I only use a single reference photo with the intention of "learning to model what I see". So yes, I hid those ears ;)
@grain: Thanks for your thumbs up! Just for fun - here is the picture where I was four weeks ago (it's my first 2h session ;) ) - and the wireframe model (3x2h from ground up). I didn't "perfect" it, because of my 2h limit, so I know there are many n-gons especially around the sides, back and neck.