I've not seen FEC on downstream on a HG612 when it's been fastpath on the downstream, only upstream.
Regarding 'opmode', it identifies the current type of connection, usually it would say 'Opmode=ITU G.993.2(VDSL2)', but appears to go blank when G.INP is present (a bug).
Regarding upstream being 19999Kbps, I think it depends purely on the modem chipset. In this case the ASUS on this connection hasn't been able to get the full 20000Kbps, though 1Kbps is hardly going to make much difference
The person has had no CRC errors, yet appears to be on fastpath, in the uptime it has had so far (days). That concludes to me that G.INP must be active on his line.