a) You should not have any problems with NOD32 and CFP (Comodo Firewall Pro).
b) If you download the latest release version, you do not have to activate the product after installation any more. This separate activation has been removed, due to numerous issues, and is much more streamlined. I’m not aware of any user having any activation issues with the latest release.
c) FREE as a bird! Although the name has changed from Comodo Personal Firewall to Comodo Firewall Pro, the product is still free, for life. There are no paid versions of CFP, what you download is the best they’ve got (and IMHO, the best - period!)