So I have done a little more digging and a little more testing; and haven't resolved the issues above.
The two clients mentioned; SCCM-HWL-D1 and SCCM-HWL-L1 are connected to / managed by a Secondary Site, the first in the SCCM deployment.
I have a third machine, SCCM-HWL-L2 that I connected to the Primary Site that is the parent of the Secondary Site, and I have no issues with that client - and it is currently using the Secondary Site as its' management point. I can remotely connect to the machine for remote assistance / RDP; but Resource and Hardware Inventory still hasn't refreshed with data yet.
Wondering if it is something in the configuration maybe..?