Thanks @johnpoz for the pointer to the packet capture feature. I know what happened now and short answer is yes, there was another DHCP server on the network, and once it was turned off, things work as expected.
Everything with this was a clean install, including VirtualBox 5.1.12r112440
Packet capture showed a response from 10.0.0.100 beating in the response from 10.0.0.1 each time. DHCP responses from 10.0.0.100 were indeed missing the DNS and Router options followed by a response from 10.0.0.1 (attached) with all the information as expected.
Solution:
Virtualbox seems to not honor turning off the default DHCP server until the entire program is restarted. I verified that the settings were 'DHCP server off', applied, NULL'd out the server settings, applied; only until a full restart of the program did the settings take effect and the 10.0.0.100 server stopped responding. If that's in the VirtualBox documentation I missed it and will double check there before taking the issue to that community.
Thanks again.

