“The questions are whether (1) his mother took any provable steps in Canada to drop her US citizenship...”
Hard to prove a negative. Do you have any evidence she did?
It’s not a negative. It would be supported in Canada by some office showing up with paperwork.
Circumstantial evidence would be failure to file IRS tax forms each year as if she no longer had to, or failure to file for baby Ted’s CRBA or birth passport to verify his US citizen status.