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.