What's curious is the genetic factor. The monotone son sang just like his monotone father. The other son (different father) is a musical prodigy and excellent singer. Both had me for a mother, both were always sung to and sung with and had therefore much the same musical nurture, different nature.

To overcome that genetic predisposition was not something the one son cared enough about to attempt to fix, and I'm not convinced it would be fixable. Your techniques and tricks with motivated students may indeed improve and correct the problems with singers without the hereditary predisposition or with marginal pitch issues, but I remain skeptical regarding my own experiences.