Words ending with “ing” are never finite verbs. I don’t even know of one exception (though as usual I wouldn’t be shocked if there is a rare exception).
Also any word immediately after “to” isn’t a finite verb. After “to” you’ll find either an infinitive or the rest of a prepositional phrase. (There are probably exceptions for what comes after “to”, like I know sometimes prepositions can sometimes be split up from their objects. I’d be interested if anyone has an example with a finite verb after “to” though.)
So “the actors pretending to be judges” cannot be a clause because there’s definitely no finite verb, so “for” has to be a preposition not a conjunction.
There are other ways to figure it out like considering meaning, but analysis of parts of speech is generally a good starting point: it tends to be relatively simple with clearer rules than other analysis, and sometimes gives clear, definitive answers.
In general, I’d be very cautious about adding finite verbs as implied words. I think they’re rarely omitted because they have a starring role.