I can try to answer some if not all your questions to the best of my knowledge as someone who also got more into the Fediverse more recently.
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When you mean follow a topic, do you mean like how on Reddit you just follow a subreddit? On what you would call a subbreddit is what on Kbin are called Magazines and is how you follow a topic. I'm not sure if that answers your question though.
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Federating and De-federating is in regards to Instances/servers closing themselves off to other instances/servers. An example of this currently is Beehaw.org closing itself off to lemmy.world and sh.itjust.works. What this means is if you are for example a user who account is on lemmy.world and try to look at a community on beehaw.org like !technology.beehaw.org, you will see the community and its post but you'll only be looking at old cached posts from before the de-federation between the instances/servers happened. This same thing goes for Beehaw because de-federating goes both ways.
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There is nothing wrong with having multiple accounts across different platforms but instances might be a little much unless you're in an instance who is de-federated from an instance you want to interact with. But of course that's all down to choice.
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This is something that a lot of people want but currently is not possible. Hopefully in the future something like that is possible but for now the only you can do (which is something I do with different fediverse platforms) is link in your bios the different places you have accounts. Not everybody looks at peoples accounts though but its a start.
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It can be yes if thats what you prefer. I personally like consuming the content on their respective platforms i.e posts from Mastodon on Mastodon, posts on Kbin on Kbin, etc because I prefer using them with their own U.I's. But if you prefer to keeps central on one account you can very do that because thats the power of the fediverse/ActivityPub.
I hope I didn't ramble on too much and its readable/understandably to read. I'm not an expert or super knowledgeable person on fediverse stuff but I try.
While I agree overall, I definitely agree with that last sentence. As someone who originally joined the Fediverse for the first in March via Mastodon, I've since then spread out and tried the many other fedi products (is that even the right word to use?) and have since settled on a Misskey forked server called Foundkey for my "Twitter" experience. While I agree with others that Threads being part of the Fediverse is good, I also agree in that the familiarity with Meta/Facebook will make people not want to branch out and explore what else the Fediverse has to offer which in turn hurts everyone else.