Because of the unfortunate centralisation that's happened, with so many people being directed to lemmy.world or choosing to go there, a big DDoS attack on world would disrupt a lot of communities that people on other instances use. It's in everyone's interests to spread the load more, which is why I find it sad when some communities migrate to world. UnixPorn was one, although it was already on lemmy.ml which is a larger instance too
How do you imagine them working if their home instance is down? You can only see the cache of old posts and your comments won't propagate outside your own instance. If LW is down, everything on LW is down effectively.
ActivityPub is an email inspired P2P-like protocol. Running communities over it is a hack.
Not in the way you think. Yes, you can still comment and post in your own instances version, but for other instances to see it it'll have to go through lemmy.world first and forwarded on. I think it's a bit murky when it comes to comment replies, though, I believe those do get propagated directly to the instance originating the comment (i.e. if a beehaw.org person comments and you reply, they get it sent directly), but you can see how disruptive it is.
Yeah it would be really interesting if it was possible to have a merged community across several instances, like you sub to super community and it gets the information from one of the linked instances and when they're up they all keep synchronised.
If your instance went down you couldn't log on but if you could switch to another account on a different instance and access the same major communities - then when that instance comes back it catches up.