@niconiconi This is the kind of problem where I'd reach for my "layer 1 packet sniffer".
Two sfp+ cages back to back with 1:2 6 dB RF splitters between them.
One half of each RX line goes to the opposing TX, the other half to side mounted SMAs.
@niconiconi Plug into a scope, protocol analyzer, or FPGA devkit with 10G capture capabilities and go to town.
See what's *really* going on on that link.