1. We already have a solution for this with tcp/ip with resiliency in the communication chain. Make the communities duplicated across servers and any server has a copy of the community.
2. This is definitely an issue but maybe a mod would only be able to control via voting with other mods for that community across servers? Make it more democratic than autocratic? Mod actions should be public too. No working in the shadows allowed.
3. You see this in gaming. People looking for interaction all swarm to the busy servers and you'll see dozens of servers all barely in use. Maybe your login should be load balanced and redirected to low use servers.

Agree it isn't simple. "We want control without control"

1. I feel like this one is an issue either way. Even if it doesn't take out the entire community, taking out the largest community is pretty impactful. It worries me that the fediverse feels so fragile.
2. I think that case is a perfectly valid one to create a new community over. I'm not saying there should never be duplicates, just that we shouldn't have them without a reason.
3. Yeaaaah, I think defederation should be handled better and admins need more granular options so that they don't have to defederate except in the most extreme cases. The fact that some of the biggest instances can't be seen by some other instances (or at least one other) is weird and worrisome.
4. I don't think this would be a reason to avoid smaller instances, but admittedly people will generally create communities on their instance. I don't think you even can create a community on another instance? You have to have someone on that instance create it and set you as a mod.