Can 'tiel' modify 'perfekte' here? I tried "He copied the actor's gestures and imitated him so perfectly", but got it wrong. So how would my sentence be expressed?
Yes. In fact, I think you capture the sense of the Esperanto better than the English sentence in the OP above. I hope you reported it.
So is the Esperanto closer in meaning to the subject of the sentence having perfectly imitated the actor's gestures rather than having perfectly imitated the actor full stop ('iel imitis lin' sed ne 'ĉiel imitis lin'?)?
The quibble seems to be that it could mean that he imitated the actor so perfectly (by copying his gestures) or that "in this way" (copying the gestures) he perfectly imitated him.
Apparently it did get reported because it was accepted 9 Sep 2019.
I don't get whether it means how he did it or how perfectly he did it
i.e. 'he copied the gestures of the actor and therefore imitated him perfectly'
This sounds like something from an anime, by copying someones guestures perfectly, he becomes indistinguishable from the person.