Tracking down all of these would be quite an undertaking. If you can find someone on PTP, KG, and CG that would probably speed. Some stuff might exist on DDL somewhere as well.
You can definitely join one that doesn't mind you signing up through a VPN. There are a handful that are picky, of course. I'm mostly fine with those that do have my real IP logged. The reality is that they can't prove it was me because someone from my IP happened to sign up to some site months or even years ago.
HDR works just fine in both, yeah. Obviously, avoid transcoding if you can. Shouldn't be a problem if everything is on your local network.