The down detection is working well, but it seems to have had some sort of unintended side-effect of making the internal server "lag out" after hours of running. The program seems to quit feeding SB proxies, and the Link Extractor instances just come to a halt. This is to be expected on public proxies (without GSA PS), as they go down and SB can't pull any working ones. However, one of the major benefits of using GSA PS + SB is to avoid this issue.
While investigating, I found a weird thing in the TCPView. Usually "localhost" is never shown in the connection manager from what I've seen (it's not a "remote address"). I think whatever happened here, is that GSA PS is "lagging out" somehow. It could also possibly be SB lagging out, but I ran this exact same setup weeks ago and never saw this issue with GSA PS. It only happened with public proxies without GSA PS.
When I logged into the server today, all instances of SB Link Extractor had stopped moving, as if there were no working proxies.. but there were 50k working in GSA PS. I often check the connections to clear them out using TCPView, but I've never seen "localhost" actually be listed in the connections.