Looking at the DHCP issue first, presumably there is a router or switch between the sites? If so, and an IP Helper is configured to pass DHCP requests, remove it. That will prevent PCs in one site ...

Currently I have DNS/DHCP running on Mac OS X Server. It's always been somewhat flakey, especially DHCP. So I'm debating running one/both of the services elsewhere. I have a nice pfsense appliance ...