I know a few of these and can speculate a bit on the others to try to give an answer before someone else replies.

1. I believe that they just disappear so you want to be sure to choose a home instance you expect to stick around. In the long term if Lemmy develops a way to "link" communities between instances then it would be a way to prevent communities from disappearing in those cases.

2. Yes they can have the same username but for your home instance you see the name like "gamer_guy123" without the @lemmy.world but on another instance they would be "gamer_[email protected]". Just like how email works they can have the same username but the domain is different.

3. An instance will limit the number of registrations to keep the bandwidth costs down. A lot of these people hosting are hobbyists but believe strongly that Lemmy is the future and are covering the costs which with limited registrations would not be massively expensive. I do wonder how much storage is required though.

4. A corporate could spin up a Lemmy instance which would likely be a more stable home instance. Since Lemmy is federated though all the activity can be synced between instances and each instance would be able to control their API so the corporate could lock down the API potentially but other instances would still be able to use their APIs. The concern I would have here is that a single Lemmy instance becomes a walled garden and doesn't federate. That would give them the same control as Reddit. It's up to users to make sure not to centralize under one instance.

5. Your home server can block instances if desired. It's important to have a home instance with a policy you agree with. For lemmy.world you can see the list here: https://lemmy.world/instances right now there is only 1 blocked instance.

Hope this helps!

How can people afford to host an instance? Aren’t there costs to hosting a server?

my instance (currently only 18 users) is on a basic server plan costing me +- $7,- a month. When that's becoming much higher I'll ask for donations to help me out (and I'll be fully transparent on the current server costs and how much is donated)

I fear that these corporate instances will be the only ones that can handle large traffic and we're just back to Reddit.

you can imagine lemmy federation like e-mail. While there is GoogleMail or other large providers that serve a gazillion users, they don't own the whole e-mail infrastructure.

Changing your e-mail provider (and thus the address) is always a hassle, more or less. But it's perfectly doable.

How can people afford to host an instance? Aren’t there costs to hosting a server?

You can take a look at open collective
Mastodon/Lemmy.world
Beehaw
#lemmy

Commenting here so i can track the responses, really good points.

Can people on other instances use your username? Could others tell which is which in comments/posts?

I think it's possible and they can tell which instance you are from by looking at username like [email protected]