a) The hack was affecting the Nokia bulletin board and some other services, but as far as I know, the login for the Ovi Publish site is a different one and is most likely unaffected. But you should always be vary about what information you give away on any online service, regardless of their security history.
b) That is no problem. In fact, my Journey Planner app is still in Extras testing waiting for votes half a year after pushing. Publishing to Ovi took about a week or two. I also have a paid GPL licensed game, Nanoparticles, that has been published fine. You have to adhere to the QA criteria, of course, so I would encourage you to read through the Ovi Store Guidelines first. That way you won't have to upload the app again if anything's missing.
c) I think you need to reply within a week's notice. For my apps, however, there have been at most 2-3 inquiries during the past six months. And for the most part I find it nice to hear from people using my apps. My apps haven't seen that many downloads yet, so the amount of support requests will probably differ if you have extremely popular apps.