I only have the G16S service manual as well, but I'm certain the Dolby C version and S version are going to be the same when it comes to erasure and bias circuits, as you switch NR off to adjust them anyway.
If all 16 channels are failing to erase properly, there are two things I would look at. The first is mechanical... adequate tape tension/contact with the erase head, and position of the erase head. Is it aligned correctly? And the obvious gunked up dirty head of any kind, R/P or erase. Anything that interferes with tape to head contact is going to reduce performance.
The second thing to look at (in any multitrack) is the main Erase/Bias PCB. Each of the 16 R/P Amp cards has its own erase and bias circuitry and adjustment points, but there is a main erase/bias PCB that feeds all of the R/P cards. You could have some component failure there or even further back to the power supply that feeds the main erase/bias circuitry. It could even go all the way back to the main power supply for the deck. At any rate it has to be a broader issue because The likelihood of all 16 R/P cards failing in that way all at once is zero.
These machines are getting on in years, so things do fail, but just a little tip I try to pass along when these things come up. If you're using +9 class tapes, like ATR, SM900, GP9, etc, on these machines you're going to burn the above circuitry up a lot sooner... because your pushing them to the bleeding edge. Don't know what tape you're using or what tape a previous owner may have used, but just thought I should throw that bit of info in for good measure and for future reference for anyone else that may come across this thread. Fostex recommends 456 for those machines and to keep these things running well into the future that's what we should be using (or bias compatible, SM911, SM468, 3M 986, etc)