Well, I have most of the write issues fixed. Don't know how. I removed a metal shield to try and see if I could get a look at the T0 sensor (can't,) and skipped putting it back on while doing a write. Mainly to see if I could spot the T0 sensor then (nope.) Only, the write was going through just fine (verify stating OK - yes a G64 with mastering data.) Did a write of the RAW stream files after that, and they wrote just fine as well (mostly, 1 error cropped up, but a second write to that section with erase turned off fixed that.) Next, I created a G64 without mastering (same disk image,) and wrote it. Verify was disabled, but it wrote just fine (did a dump with no errors.) Tried a G64 with some home written software, and it worked just fine as well. Tried this specific D64 from a 3rd party software purchase, and it's nothing but errors.
Thought at first it was this software, somehow, but couldn't figure out how a D64 without an error map resulted in a disk of nothing but errors. Then I tried creating a blank G64 and writing that and got nothing but errors. Next I created a G64 with 64-Copy instead of Vice, and it worked just fine. Used 64-Copy to convert the D64 to G64, and it wrote just fine as well.
Further experimentation showed that formatting a G64 disk, even when created elsewhere, caused the same problem. But creating a blank disk in 64-Copy then just copying files over until it's full doesn't cause a problem. Doing a disk copy will also result in nothing but errors.
My final test run was to do everything in a different emulator. Ended up with the same results. Doing a format or data level disk copy breaks the G64 for DTC writing, but just copying over files seems to work just fine. So, writing a game's data disk that I've been playing in emulation to continue my game on real HW will probably be hit-or-miss. Would depend on how low a level the game writes to the disk
If it's an unprotected disk, then I could just use D64 and do a conversion when the time comes.
that is all a change of topic, though. So, to get back "on" topic:
Can't visualize the T0 sensor, so I don't know if it has a good extension added on or not
Guess I'm just going to have to live with shoving the head back past track 0 when something causes it to stop while below 0