4. Yes you cand, but is not necessary. When the PDC fails, BDC will hold the domain up (authenticate clients, allow access to resources, and so on). The problem is that BDC only has a read-only copy of SAM (which is your user and domain resourcesaccounts database). You cannot add users while only the BDC is up. If you think the PDC is faulty and you cannot recover, you can promote BDC to PDC, but you can't re-use your PDC again (you will have to re-install windows NT). If you think PDC could be fixed, run only with the BDC until then. If you want more details, let me know.
5. If Win2k server fails (I assume you're talking about a D C- domain controller), nothing really happens, except that resources from that server will not be availaible (printers and shares). WIN2K has only peer domain controlers, there is no BDC or PDC. That means that you can carry on and add user and resource accounts on any other domain controller (unlike NT4).
I hope this answers your 5 30 points questions..:))
Regards,
Radu