Got it! The birth date I have for Christopher Russell came from his grave marker in Houghton, Ontario. The marker is partially buried, so it doesn't show on my photo of it, but the Ontario Genealogical Society transcribed it in 2005 and they recorded that the marker is inscribed with his precise age: "78 yrs, 3 mos, 16 days." Counting back from his death date of Apr 10, 1872, we get Christmas Day of 1873! Exactly what you got -- now I'm REALLY hoping you can tell me where you found those birth & christening dates. I will go to Houghton when the weather permits, with a shovel, and see this inscription for myself.
I think this will be the definite link I need. With names & ages matching, and now a matching birth date, plus the echoed name of David's first child, I think I may lay claim to these folks with some justification. Or could it still be all just coincidence?