Hi,
You can copy test cases from Project A to Project B by going into a Project B test cycle, and selecting test cases from Project A. Something to note is that the test cases will remain in Project A - they won't 'move' into Project B, and you may find that amending the test case in Project A will prompt a warning in Project B that the test case has changed (and can be updated).
Alternatively, you could do what you're already doing (export/import the test cases) but remove the folder data which I assume you could do by removing that string with a word editor via Find/Replace.
Your second question is almost answered by my first suggestion above - you can reference a master/central project of test cases, but they can only be referenced in test cycles which you said you don't want to happen.
