What struck me on hearing about this is that black people use that word themselves. That means that the word, in itself, is not inherently so offensive that it should never be used.
So if, on hearing that word, the hearer has to check the skin colour of the person using it to decide whether to take offence, then there is a degree of choice involved in that offence. Taking offence is not an uncontrollable response, even if there is an negative or shocked automatic reaction. Ultimately then, whether offence is taken ought to come down to intent.
I have read that the black actors on the stage looked startled, but did not otherwise respond. In a perfect world, they might have been ready with a witty retort that would have defused the situation, however not responding negatively is enough. If that’s true, then good for them.
It sounds to me as if others have taken offence on their behalf, which is incredibly unhelpful and does nothing to make anti-racism campaigners look reasonable and (sadly) makes them simply look prejudiced against those with disabilities.
This is somewhat analogous to the whole hate crime/non crime hate incidents situation. Some people who take offence are unreasonable and assuming an offence has been committed based on someone reporting that they feel offended is open to abuse. There has to be acknowledgement within society regarding how a theoretical rational person would respond. Otherwise we descend into madness as the perpetually offended bully the rest of society into silence.