1. You can renew your developer certs once the old one has expired. You also need to update your provisioning profiles with the new certificate.
2. No need to republish any live apps. The only possible problem scenario is with Adhoc apps built with an expired provisioning profile...they will not run any more. They will to be rebuilt with a valid cert. Live store apps are fine.
3. Yes, seems every year you need to renew certificates. Also if you change dev machines you might need to renew if you can't transfer your certificates.. no big deal.