Hello and welcome! I'm on a similar quest, let's share! First, what I know:

• Can someone register with my username on other instances, or is my username “taken” across the fediverse?

Yes, they can. We need to understand that 'names' include the instance here. My username is not Spzi, but @Spzi@lemm.ee. (Same story with communities. There are no 'same name' communities on multiple instances once you accept the instance is part of the full name). This identifier is unique per instance. Further, you can choose a display name, even if it conflicts with another display name from the same instance (AFAIK).

You can register your name on other instances, to block it from being taken by others.

• If so, how are spoofing and/or scams using my username but on a different instance handled?

Maybe we use this platform in very different ways, but I wouldn't worry about this too much. In contrast to mastodon, we follow topics, not people. In contrast to reddit, there is no account karma. If you still run into trouble related to spoofing and/or scams, just remember to look at the full identifier, not just parts of it.

What happens to accounts, communities and posts when its instance decides it’s been a fun few months and decides to shut down?

I'm not exactly sure. Please someone @notify me when someone else has a good answer.

it’s not my data that is federated, but only links to the data in other instances. Would that be a good understanding on how the federation technically works?

AFAIK, data is actually cloned in the process of federation. Edits and deletes are propagated. Each instance holds a local copy. Threads can have differences when viewed from different instances.

This post and subsequent comment was gold for me to learn more about it: https://lemmy.world/comment/205763

1. They go bye bye. See all the Mastodon closures. Although Mastodon has account migration, not sure about Lemmy.

2. Yes and No. They can be @durd_ but they cannot be @durd_@lemmy.world.

3. See 2.

Your data is federated as well, not only links, but your home instance is the only true copy and instructs other servers what they should do with your data. Which leads me to believe your posts and comments might not be gone when an instance shuts down. But your ability to login with your account is definitely gone.