According to the DSON Installation instructions, the DSON core should be installed to the Poser Pro program folder, not an external runtime. Poser isn’t always happy with python files installed in an external runtime.
Because the Poser companion files install a bunch of DS structure and files Poser doesn’t need, I preferred to install the Genesis files to the DS content folder and map that runtime in Poser. But that’s me. I am just happier with a cleaner Poser runtime.
They don’t have to be in the same Runtime, but both Runtimes need to be mapped in Poser as external Runtimes. So I would install the DSON Core to the DS4.5 content folder and map that in Poser, and install the Poser Companion Files in a different runtime and map that also in Poser.
No, the DSON Importer needs to be installed to the program folder—that’s the plugin itself. The DSON Core is the content, and must be installed to a writeable folder.