Yep, you can use the same physical NIC for multiple VMs. The NIC and Hyper-V will figure out the subinterfaces etc. as long as you setup a virtual switch.
Here's an article that explains how the virtual switching works http://www.altaro.com/hyper-v/the-hyper-v-virtual-switch-explained-part-1/
I would not try to run the host as a DC. Much better to leave the host alone and run the DC and a virtual router (which you will need to run the domain in its own IP subnet) as vms.
If you do add a second NIC to the host, reserve one for the host and one for the virtual switch. If you only have one NIC, they must share it. If you multihome the host it is yet another reason to not run it as a DC!