here's a cohertly sound Answer.

1. What happens to the communities/comments/accounts if a Lemmy instance goes down? Do they just disappear?
   When a specific Lemmy instance goes down, local users won't access their accounts, communities, or comments until it's restored. The data seems to "disappear" but it's not lost if the instance comes back online. Content copies exist in other federated instances but the original data is tied to the creating instance. BTW, you can backup your toots, comments and anything else on your account on your current instance and start again

2. Can people on other instances use your username? Could others tell which is which in comments/posts?
   Yes, usernames are instance-specific, so the same username can be used across different instances. However, usernames include the instance, making identification clear. For instance, 'username@instance1' and 'username@instance2' indicate different users.

3. How can people afford to host an instance? Aren't there costs to hosting a server?
   Indeed, hosting an instance involves costs for server, bandwidth, and potentially maintenance. Individuals hosting instances usually cover these costs themselves or use donations or sponsorships.

4. Is there anything stopping corporate interests from hosting a Lemmy? I fear that these corporate instances will be the only ones that can handle large traffic and we're just back to Reddit.
   Theoretically, a corporation can host a Lemmy instance. But federated platforms like Lemmy ensure that no single instance controls the entire network. Even with a popular corporate instance, users can choose other instances or create their own, allowing diverse moderation policies and community norms.

5. Can an instance go from fully federated to partially without telling its users? How would they know?
   An instance changing its federation policy can impact the available content and the reach of users' posts. Although there's no built-in notification system for such changes, a responsible administrator should inform the community, potentially using the instance rules listed in the sidebar or other official communication channels. Users may notice a change if they stop seeing content from certain instances, or if their posts aren't visible on instances they used to federate with. Such a shift in federation policy could also alter the dynamics of moderation and community interaction on the instance. you can see what instances is blocked on /instances. and /modlog shows all moderations.