The best method is to keep everything in one catalog. Not only is this less work, but it avoids the difficulties you are having, and it has 22 bazillion other advantages as well.
I'm performing some tests here, but it seems that Lightroom only imports smart collections. All the normal collections should be re-created manually [apparently, the duplicate issue was some mistake of mine, but I'll keep testing to see how it's going]
I'm performing some tests here, but it seems that Lightroom only imports smart collections.
Incorrect. All collections, smart and regular, can be transferred to another catalog via the menu command File->Import from Another Catalog.
I've tried twice here, but only collection sets including smart collections were imported. There was a single collection set including some regular collections that weren't imported