Hi Mackenzie — welcome to Medium! I’m not sure where it says you can’t publish posts from elsewhere — you can and Medium even has an import tool and a help file about how to do that. That said — I don’t recommend it. Seems that when people use the import tool, they get thrown in curation jail and Medium doesn’t feature their posts. What I’d suggest is to use good old fashioned copy & paste instead. It doesn’t seem to trigger the same flags.
With the economic theory vs. new tech issue — you’d be surprised how many areas that same kind of discrepancy exists in. Or maybe not. You might need to lean heavily on tags and topics more than publications, if that makes sense. People, even. If you find the people who write on similar topics, and interact with their posts, you’ll inevitably find the right audience, too.
On publications — yes, you can start your own and also publish in others. Because you’re dead on right — a new writer with a new publication is going to be hard pressed to get reads. But if you juggle both for a while, your publication will grow.
I agree that a publication with more than one editor has a lot of merits. I think the best way to find the people that would be interesting in working together would be to dig around to find people who write on similar topics. One of the unfortunate things I see here is that it’s hard to get traction when you’re new. It’s like the law of numbers kicks in (social proof) and it’s a lot easier to get people to team up once you get some traction yourself.
That said, I have no doubt you will — a lot of people here are floundering on what lane they want to drive in and you’re not among those. I think the clarity you have will help you moving forward.
Hope that helped (at least some) and nice to meet you! :)