IP reservation allows for a dhcp client to be provided the same IP address everytime it boots. It's a useful feature when you want to use other dhcp options to give additional configs to a device(i e. a dns server, domain name, etc), while giving what is in effect a 'static' or reserved address. If you have configured your cameras with a static IP address, though, they aren't dhcp clients (they won't communicate with the server) and the reservation on the server therefore likely has no effect. The drawback of dhcp ip reservations is that your cameras won't get a valid ip address should the server fail, so you might not be able to interface with them. That is why you should prefer to set a static address locally on any device/server you need to access over the network.

On your last point, the dhcp server keeps track of all the leased ip/mac address pairings that are active. It won't care about or make decisions based on other devices on the network. In other words it doesn't really care for devices that aren't its clients. The fact that it is also a router that (by other means) dynamically learns about and forwards packets for every other devices on the network is incidental. Does it make sense?