the trouble with informants

My friend Ingrid is in Berlin. We were going to go to Badeschiff, a swimming pool-cum-sauna floating in the Spree, but she had to see her doctor and it was too much. So we met at Kleisther, a café down the street from my new apartment which has a Hotspot.

I had a grammatical question. Words like 'Wohnung' and 'Nutzung' are, of course, feminine, so the genitive looks just like the nominative. But if they're put in a compound an s is added: 'Nutzungsdauer'. What's going on? Other things being equal, I would have thought the compound required the genitive form (cf Altertumswissenschaft), but this isn't actually the genitive, so wha-?

Informant: Hm. I don't know.

We discussed an exhibition Ingrid had seen on political minimalism. I had another grammatical question (it's hell being an informant).

Why do you say KotbussER Tor but SchlesischES Tor? Tor is neuter, so Schlesisches Tor makes sense, but why is it Kotbusser Tor?

The informant mulls this over. She says: Well, Kotbuss is a town, Potsdam is a town (Potsdamerplatz), if you were putting Köln in a compound you'd say Kölnerstraße. Dusseldorferstraße. But it's Kleistpark and Alexanderplatz without an ending because they're people, Schlesisches Tor, well, schlesen is a verb.

This reminds me strangely of an episode in Winnie the Pooh.

Christopher Robin explains that the name of his bear is Winnie the Pooh.
Narrator: But I thought Winnie was a girl's name?
Christopher Robin: But it's Winnie THER Pooh.
Narrator: Ah. NOW I see.

(Note to American readers: THER: Brit for Thuh. The r is silent. Similarly, when the girls in Little Women call their mother Marmee, the pronunciation is, mirabile dictu, Mommy. The little women have a Boston accent. We thought you should be told.)

I say: OK, well, what about the sechsigER Jahre? (i.e. the 60s) Why is it 'er'?

Informant: Well, if you were talking about 60 years you'd say sechsig Jahre, but here you're talking about something in the past.

I say: So you wouldn't refer to the 60s of the 21st century that way? You wouldn't say 'In den sechsiger Jahren des 21. Jahrhunderts'?

Informant: It would sound funny. Maybe because we're used to using it of the past. You might say that, but it would sound funny.

I'm disheartened, demoralised. I completely forget to ask my informant whether she shares my instincts re the Australopithecus effect.