Lemmy and Kbin are very similar, it’s reasonable to be confused. Very important, Lemmy instances and Kbin instances can interact with each other’s content, so whatever your choice ends up being won’t be about content since they have effectively the same content.

The difference between the two lies in some behavior differences (for instance, Lemmy intentionally doesn’t have a karma system, and Kbin does), in their very different web UXs, and since their APIs are different, the native apps talking to each are also different apps.

There’s also an ideological aspect, the creators of Lemmy initially created it because they were finding Reddit content to be too US-centric. Some say that they support communist dictatorships, but I’ve looked a bit, and evidence of that is flimsy, it looks more like they decided not to suppress anti-American content on their instance, and people may have extrapolated a bit. Or maybe there is truth to the rumor and it’s just well-hidden? 🤔 I believe the creator of Kbin started it because he disagreed with what he understood as the ideology of Lemmy’s creators.

I’m on Lemmy because it and its apps are more mature, and I’m not convinced by karma systems and what they may incentivize. And because I don’t find the case for the creators being extremists to be very convincing, from what I’ve seen. But Kbin looks pretty decent too.

I’m afraid I don’t don’t have answers to your other questions, but I thought I’d inform on this.

Lemmy devs are tankies(support authoritarian regimes under the guise of 'communism'): https://kbin.social/m/lemmyworld@lemmy.world/t/47012/-/comment/196579

Regardless of what method you use to import to a community or magazine eventually, start using the first steps in your linked page to export the backups of the subreddits immediately. That will give you breathing room to eventually be able to import them if you find a solution, even after the API is deprecated.

Kbin and Lemmy can access the same content. It's more a matter of which you like better. Lemmy is more popular and if most of your communities of interest are on Lemmy it will be easier to find and subscribe to them. If you have more magazines of interest on Kbin then that might be easier. Kbin calls them magazines and Lemmy calls them communities but they're the same thing.

Instances can not be merged as far as I know. An instance is a virtual or physical server that lives on the internet running the Lemmy or Kbin software. Instances synchronize automatically with other instances on the Fediverse using the ActivityPub protocol. If there is a way to merge two instances, you'd need to be the admin (owner) of both instances and do some data base gyration between them. It would be involved.

Easiest is to simply sign up with an instance that allows user community creation. In that case you can set up your community and put whatever content you want on it. I haven't looked at that importer software, but it's not really going to be a big help if it requires you to be the admin of the upload instance. Unless of course you do actually want to run your own instance.

Anyone can set up an instance, but it's not a trivial thing to do if you're not already familiar with running an internet server. You can certainly learn, but it will be a fairly involved project when starting from ground zero.

It requires access to the Lemmy instance database.