I'd say to really figure this one, I would run wireshark on two of the machines that can't talk to each other and capture on both and look at the captures to determine what is causing the issue. If you can get the capture files, you can PM them to me and I can look if you don't want them available publicly.
>>>Well, yes, that's a normal Microsoft design function; they only want you to use their OS for free networking. However, there's usually a way to do a workaround, though that usually involves building a Microsoft server such as 2008 Server RT or newer. Multi-protocol environments are then allowed to be added onto the NT kernel going all the way back to NT v3.51 as additional purchased services (more money for MS). Normal home users do not use Linux, just like they don't use VMS or Solaris. In fact Home Networking doesn't play nice with Mac iOS either, as you said they aren't designed to work together in a peer-to-peer LAN environment; they never were.Yes, the default "upgrade" pushed out to XP users is Windows 10 Home. W10 Homegroup isn't usable as it is designed to block the other OSes I use. Shares don't work properly on a mixed network if Windows is allowed to use Homegroup, but fortunately even W10 can use normal shares.
This sounds like a "Microsoft" solution - re-install all the operating systems!