Nope. Once you import a project, it becomes independent, and is not linked to the old project. AE treats projects differently from footage, in other words.
Regarding your other question, I would import project A into Project B even if A wasn’t finished. Why? Because I could always finish A inside B.
But if I had more than one person working on a job, and Person A knew Project A, then I’d let him finish it before I (Person B) imported it into my Project B.