What is a relative pronoun's referent when it follows a prepositional phrase?

Solution 1:

If I understand the question correctly, the question is which word is the head of the NP which "who" refers to.

In this case it's easy to resolve any ambiguity. You can train actors but you can't train a generation. The only way to understand the first sentence entails treating "several generations of" as a quantifier. As such, "generations" can't be that head. This implies that that head must be "actors".

Compare:

Stella Adler trained a large number of actors who include Marlon Brando and Robert De Niro.

In this variant, the head of the NP referred to be "who" can't be "number".