Yes, you can always defederate from other instances if you want. In fact, a lot of fediverse instances do just that to protect their hugboxes from the rest of us who actually value our ability to communicate with each other. And there are certainly a number of instances that defederate from others on technical grounds rather than social ones.
I'm sure there's a nicer way of saying that. I've worried that I would end up in an echo chamber of my own on there, but saying things like "hugbox" are unnecessarily rude to people who have tended to prefer isolation because of the effect the world has on them. We can't choose how others react to content, but we shouldn't chastise them for wanting to protect themselves from harm, even if we don't agree with the definition of harm.
In any case, instances full of trolls, spammers and nazis also tend to get blocked by others who prefer to communicate normally. Blocking serves a useful function. If FAANG servers show up, I'm sure some would block them out of principle. Others once they start breaking the protocol.
It isn't. "Hugbox" is used pejoratively specifically to attack people with perceived left wing beliefs and associations. The idea is that people who aren't interested in seeing a ton of transphobic content or whatever are too "soft" to expose themselves to "real conversation."
"Echo chamber" is more neutral in orientation, though still carries a negative connotation. "Hugbox" is specifically slanted.
As for not wanting to see transphobic content, I think that's entirely a reasonable position. People shouldn't tolerate abuse. Abuse != "real conversation".
People are weird.
And even apart from that, I think everybody has a right to choose who they associate with, and that does not have to include harassers. That's more relevant to social media and less to teaching classes of course, which do need to be open to everybody. But if you're choosing to follow a class with the specific purpose to harass someone, there's something deeply wrong with you.