1. They can choose who to defederate, pretty sure beehaw has already done so with lemmy.ml and others.
2. I think every instance is different but on some if you try to sub to a defederated community it’ll get stuck on ‘pending’, also you’ll probably see a lot of posts about it. It doesn’t seem to be something that’ll happen often.

Here’s an analogy, hope it helps:

Kbin.social and beehaw.org are like really advanced email servers (e.g. outlook.com, gmail.com)
The content you’re seeing on kbin is like viewing an email sent from another server, in the case of this thread you’re getting content from beehaw.org.
These aren’t just normal emails, they’re super advanced emails with comments, replies, categories, votes etc.
The content is downloaded to the kbin servers for your viewing pleasure, and you can send an ‘email’ back to beehaw by engaging in their posts/communities like you’re doing now. That will send what you’ve done back to beehaw so it’s all synced up for their users.

If beehaw ever goes down/gets unlinked from kbin you’ll still have the old ‘emails’ (content) but you won’t be able to engage with them anymore. Like if someone deleted their email account it doesn’t delete all the emails they sent to other people.

is that you Spez?