Well, I think it's important that we not conflate reasonable transgender people with radical extremists within the transgender community. I'm bitter because I've been attacked by trans people (and their "cis allies") for expressing that I wouldn't consider a romantic/sexual relationship with a trans person. And I'll continue to resist such "policing" of my life by complete strangers. But, on the other hand, I'm also not going to go around telling trans people what THEIR LIVES have been like for them, or making assumptions about how they endure a type of oppression that I personally don't face.
This is what Dave Chapelle appears to be tone-deaf and oblivious (or willfully-ignorant) to. The fact that people are outright MAKING EXCUSES FOR HIM is downright maddening and disgusting. It's emblematic of why the culture wars have gotten so bad.
I also reject the premise that Black people and women aren't given a certain level of PC "protection." Look at the shit that Sheryl Underwood and Joy Behar get away with on DAYTIME TELEVISION.
You ask how we're supposed to determine whether someone is "legitimately offended" by an attempt at humor. I say the onus is on the person who feels offended to express the burden of proof in regard to what they find so offensive. If the comedian (and their supporters) isn't willing to at least hear them out and consider a well-articulated point, then that tells me the comedian (and their acolytes) mainly only cares about their own narcissism and personal gain.