Re: the sayings - it's always fun when Germans translate German sayings literally word for word. So your 'du hast nicht alle Tassen im Schrank' would be 'you don't have all the cups in your cupboard', du hast Salat im Kopf would be 'you have salad in head' (someone said this to me once about himself when I was learning German, I was bewildered), and 'du spinnst'' would be 'you spin' (not you are spinning - Germans find getting the present continuous form correct really tricky).
"What if that's not a hobby as such, but he develops an enthusiasm for another culture/history of a place he's visiting for the first time, but has researched in exhaustive detail? I'm imagining a face-off between him and a native of the other culture, who is far less well-informed about it than he is -- is he likely to point out errors in the other man's knowledge?" - Absolutely, and he would relish doing so. He would dish out all the details he's read in his incredibly complex upmarket guide book to someone who's lived there all their lives and express faux surprise that they didn't know, but secretly be delighted. See the term Besserwisser (know it all) - it's particularly prevalent among West German men.
I used to climb with quite a few Germans, and it was noticeable that they were used to marked paths, and less used to the idea of taking a map and compass and going off-piste. Also they were used to the drier alpine weather, even the experienced ones, and frequently under-estimated the changeability of British weather, especially how a sunny day could turn into wet and cold. He would however have good quality high-tech equipment, and be a bit of a 'gear freak'. -Absolutely - it's actually much easier walking at over 2,000 metres in the Alps, with frequent path markings and signposts giving estimated timings, than anywhere in the Brecon Beacons. Your man would get incredibly frustrated at British incompetence there.
The Germans have a pretty patronising view of the British generally (but the British return the favour with their view of Germans, so hey ho). The British are all about the Queen, cool Brit music, scones, old-fashioned but cosy vibe. Germans who have recently been to the UK are truly horrified by the poverty they've seen.
You could have some fun with a German mistakenly using English words in the wrong context. For instance, the word for mobile phone is Handy. I could imagine your man saying 'where's my handy?' causing some confusion. And the word hi-fi is used, but Germans insist it's pronounced high-fee (because that's how you say high fidelity, right). Your man would insist to an English native speaker that we have it wrong!
Germans don't pronounce an English-style 'ah' sound, it tends to become 'eh'. So 'handy' would be 'hendy'. It means for example, lots of them can't pronounce (or hear) 'man' and 'men' differently.
"German scouts are Pfinders (spelling might be slightly wrong there)." - the word is Pfadfinder (literally pathfinders).
"Also, you should do some research on German academia if you want him to be a postdoc because it is significantly different from the UK/Ireland, there are many more years of study and you are a sort of employee/student usually, working for your supervisor (depending on the field)." - good point, dreaming - the main postdoctoral qualification is called a Habilitation, and it formally enables people to become professors.