The question of ambiguity with pronouns is maybe somewhat ambiguous itself
In general though, if a pronoun can plausibly refer to more than one subject it is considered ambiguous. What constitutes plausible of course allow a little wiggle room in interpretation. Let's look at your first example.1. Researchers claim to have developed new "nano-papers" incorporating tiny cellulose fibers, which THEY allege give THEM the strength of cast iron.
Could researchers have the strength of cast iron? Plausibly no, one might think, as even the strongest power lifter doesn't quite have the strength of cast iron. The thing is people can be strong. And in hundreds years, perhaps we will have the (ahem) technology to have humans with the strength of cast iron (again the comparison isn't directly apt, as cast iron strength doesn't completely relate to human strength).
However, let's compare the ambiguity to the potential ambiguity in the second sentence. 2. Confronted by radical changes in production and distribution, modern Hollywood studios are attempting various experiments in an effort to retain THEIR status as the primary arbiter of movie consumption.
Do you know any experiments that direct movies? No, the question is utterly absurd and nonsensical. Therefore, we would not suggest that experiments are trying to retain their status as the arbiter of movie consumption. It is clear that only humans create movies (vs. the first sentence that talks about strength, something displayed by both humans and metals).
Therefore, a pronoun is not ambiguous if it reasonably can refer to only one noun. And by reasonably/plausibly I mean it does not lead to arrant nonsense, the way experiments (abstract noun) starts doing things only humans do.
Hope that helps
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