Actually since the OP is using his home LAN for the Whole Home traffic, your suggested setup would function just fine for him, Whole Home as well. I ran (still run part of my receivers (5 on LAN, 7 on DECA)) on my home LAN long before I brought in SWM.
I like your suggestion actually! Using the legacy ports makes it all pretty simple. (assuming they aren't considered in the tuner count on the SWM side)
Several SWM's can be leveraged but that adds splitters, likely amplifier, and a host of wires, additional PIs, etc. Using the already present legacy ports to hook up one or two of his existing units as they already are wired for, simplifies the lot nicely. THe only add on your way with his LAN is a BS filter on the HMC so i'ts DECA output isn't flooding back through the splitters and SWM.
Of course this side steps DECA and direct TV support for Whole Home issues and the OP should consider that against the DIY approach.
Don "wouldn't have even considered those legacy ports. great call" Bolton
Since you can only have a maximum of 8 tuners per SWM port on the SWM-16, the SWM port that has the Genie can only take one HR-2x, unless you add a second with a disabled tuner as suggested.
If you didn't want to disable any tuners, and were already willing to connect one to a WB68 anyway, you could connect the Genie and one HR2x to one SWM port, 4 HR2x to the other SWM port, and have two coaxes to the remaining HR2x from the legacy ports on the SWM-16. You'd need to reconfigure that HR2x to non-SWM, and it could no longer participate in your whole home network.