The ghosts have been captured.
The real lesson here
...is to always fully research, overview, and document all new hardware in your networks.
Make time to proactively volunteer to set things up right the first time, even if it is not your sole responsibility, and even if your boss wants to setup the new devices.
Specific details
In my case one client was a laptop from a former employee, now an outside consultant, whose machine was running on the wrong LAN, and had not been properly checked to be on that network; and the other client was a phone which had also not been confirmed and checked-in to my hardware lists. (The comment that both MAC numbers were iMacs was a mistake).
My method of clearing DHCP on OSX Server, was correct; I've cited it again below.
In regards to how manage DHCP on OSX & Darwin, especially on the command line, read on--I've self-answered and left a small trail of interesting links:
OSX DHCP Overview
The quintessential guie to Mac OS X Server Command-Line Administration, which is directly from Apple, albeit from a previous version of OSX, contains enough information about managing Macs from the command line to keep you busy reading through an entire vacation.
If that is not enough, there is also a massive article on OSX DHCP at Krypted.com.
Turning off DHCP (several methods are described just below) then deleting or editing /var/db/dhcpd_leases and turning back on the service, is indeed the best way to clear the leases.
You can turn off and on DHCP using the giant switch icon on the DHCP tab, at the upper-right of the screen, in the GUI of the current version of Server.app.
Alternatively, on the command line, you can execute (likely as root):
serveradmin stop dhcp to stop DHCP and serveradmin start dhcp.
You can get loads of information about DHCP by typing:
serveradmin fullstatus dhcp
A Note about Bootstrap Protocol
For your log-reading pleasure, it is also important to know that Bootpd handles bootp instructions (internet bootstrap protocol), which is intertwined with DHCP, and the community of Wikipedians had the best definition I saw after a bit of searching about this relationship:
"... The Bootstrap Protocol (BOOTP) is a computer networking protocol used in Internet Protocol networks to automatically assign an IP address to network devices from a configuration server... While some parts of BOOTP have been effectively superseded by the Dynamic Host Configuration Protocol (DHCP), which adds the feature of leases, parts of BOOTP are used to provide service to the DHCP protocol. DHCP servers also provide the legacy BOOTP functionality."
This Technet article from Microsoft has also a very interesting comparison of the relationship between these two daemons, as well, for an additional resource.
You may also be able to use launchd, in a pinch, to essentially disable and re-enable DHCP, since DHCP depends on its services, for example, it's particular leases are stored at /etc/bootpd.leases, but contrary to old information floating around the web, this will not necessarily clear current versions of OSX Server DHCP information.
For example:
sudo /bin/launchctl unload -w /System/Library/LaunchDaemons/bootps.plist
followed by sudo /bin/launchctl load -w /System/Library/LaunchDaemons/bootps.plist will restart the bootstrap service.
