We have success! Three BMs in the toilet in a row with no accidents. After almost a year of careful, gradual work, we made it.
I'm sure we will still have a few regressions and it will be interesting to see if he continues to use the toilet without the restrictions and incentives, but I really think we've crossed the barrier.
With one completely trained, that should give me the mental energy to return to training the older one. Maybe I can make it gradual enough to move past his indifference.
Either way, I'm proud of myself and of my son.