"If people are willing to live as legal or illegal immigrants, with rights that range from limited to none, then logically, they should be even more eager to move to a Romerplex, which would promise most of the economic gains of uprooting to another continent while allowing migrants to stay closer to their families and cultures."

I think this is one of the stronger arguments- given that's it's better than living as an illegal immigrant in a far away nation, some people in their current living situation would choose this. I definitely agree with you that differences as a result of culture are huge and not to be underestimated. But a lot of these countries are clearly desperate enough to try stuff (similar the Green Revolution when India had problems with extreme hunger), and it's conceivable that this could work.