I guess it’s only a matter of time before more people hop in and publish their findings. The research you’ve done and shared so far has helped a lot.
It just came to my mind that the larger-than-actual-capacity values in 0x68, 0x6C, 0x80 could be due to each sector’s overhead (GAP, Sync, Addy Mark headers and ECC). See http://www.seagate.com/tech-insights/advanced-format-4k-sector-hard-drives-master-ti/. There’s also some books written about this. I’m also making a wild guess that these larger 32 bit values before the defects were cleared represent the total space used (default allocated sectors + spare sectors). So when more spares are used, the larger it becomes.
I’ve been trying to port my APM prog to DOS/Win32 the last few days. Managed to get an EXE file but the IOCTL portion failed to function - apparently a recode is required. Otherwise we can have a Win32/DOS open source to dump the mods 
I got the source from hdparm 6.9 and managed to compile into EXE and run it. hdparm 9.x (and probably versions after 6.9) uses sgio (sg = SCSI Generic) - this probably explains why no one made any Win32 versions after that.
Update : The sgio.c looks very similar to scsiata.cpp from the smartmontools project. Let me see if I can utilize scsiata.cpp to port this to Windows or the sg3_utils
