I am positive the info was going to my machine. The download was pegged at the top speed my WHS server can send over my powerline network adapter, and there was zero upload speed.
I have a theory. It could have been indexing running amok. It was not set to explicitly scan network folders but when I trimmed the index locations to just user files and did a couple other housekeeping items, it stopped and the logfile stopped getting jammed too.
The logfile entries were:
504 stordiag, completing a failed non-read write SCSI SRB request
507 stordiag, completing a failed IOCTL request
No freaking idea what those mean, but that was the logfile being slammed with repeated entries for multiple device IDs by the system process that was connecting to my WHS IP address. It took a freaking hour to chase all that down with process IDs correlating bytes written with network traffic rates because the process was named just "system", but I was able to 100% certainty pin it down that when the network was flooded with the transfer from my WHS computer to my win10 computer, that logfile was the only file with a huge amount of traffic to it. When the network flood stopped, the logfile quit being modified.
I changed the WHS backup settings to ignore the 2 small system partitions on my SSD (boot and restore I suppose), and only backup the C: drive. That might have also had something to do with it, maybe failures to access the 2 small partitions were freaking out the computer.
All signs of an immature OS, but that's what I get for installing it on day one and putting it on the same LAN as an old WHS box. Still, you'd think that MS would have tested this configuration, upgrading a win7 machine that was using the original WHS OS and backup software. Now that I think I've beaten it to death, it all *just works* again, in spite of everyone scoffing at me for not using their favorite *nix based server.