@mannis79
A) Yes, that's right. If you don't want the cameras split I think your only choice would be to connect ALL cameras to one or more PoE switches.
B) Yes, you are correct, although some camera functions may rely on internet access, e.g. Network Time Protocol (NTP) to do time sync/daylight savings etc, and some cameras do their own alerting or uploading of captured images. e.g. I have ANPR cameras which do FTP upload of images and I have not found a way to have that work if they are directly connected to the NVR. So what you require may depend on what cameras you have and what functions of those cameras you will be using. If you have a decent router with blocking/firewall features you should be able to setup rules to only allow the traffic you require?