It seems that in "men's toilet" and "teachers' seminar,"
whom it's for takes precedence over
what it's for, therefore,
possession. I think in both cases the nouns adaquately describe the purposes (toilet/seminar), although the second is probably more arguable.
Regarding the second issue, my sense is that both may be used, depending on what the author wishes to stress.
They decided to raise executive pay and cut worker pay.
They gave the workers a better health plan, but then they cut the workers' pay to make up for it.
- A.
Edit. In the first sentence, "takes precedence" is confusing. I think the toilet belongs to the men and the seminar belongs to the teachers. As you point out later, if "teacher" were a descriptor, we'd say "teacher seminar." (Is it your understanding that this applies only when the descriptor is an adjective in the traditional sense?)
Edit Edit. Well, looks like I'm wrong again. If we would say,
toilet for men / seminar for teachers, we call it
purpose. If we would say
toilet of men / seminar of teachers, we call it
possession. Again, the second one is arguable. It would be more (nearly) clear if we were talking about "college." In the British sense, "college
of women/teachers." (possession) In the American sense, "college
for women/teachers." (purpose)
Note. My understanding is that an American college is an institution of learning while a British college is an organization of students to facilitate their learning goals.