Thank you kryo, but they were unable to figure it out either.
While troubleshooting (read: trying to figure what was blocking the acceptance of other PCs to join games hosted on this box), I used Autoruns (a Microsoft SysInternals tool) to disable anything autostarting during a boot that I didn't recognize. Unfortunately, I apparently didn't recognize the USB and Ethernet drivers. Oops. Since my motherboard doesn't have a PS/2 port, the only options I have for controlling the system are USB keyboards and mice or a remote desktop connection via MSTSC. And since the USB ports were all disabled and the ethernet was disabled ... I had screwed myself.
Luckily all data was on drive D, so I could do a fresh Win8.1 install on C: w/o losing much of anything data-wise. I then reinstalled my apps and ... I can now host SINS (and Rise of Flight) games from this machine w/o issue.
No idea why it works now and it didn't before. <shrug>
Thank you for replying tho! You're the only one who acknowledged my plea for guidance, it's greatly appreciated!