UPDATE - close the circle.
We made it to Whitby, ON last night via MAN-PHL-BUF and the Queenston Bridge into Canada.
No issues with CBP or CBSA. I was expecting to have to go "inside" at the Canadian Border but was stamped in at primary without having to get out of the car.
Only slightly annoying bit was checking in at MAN for PHL when officious ICTS agent working for AA tried to insist my wife needed an ESTA with her Canadian passport. MAN seems to specialise in this kind of individual at the moment (see my recent "e-gates" post...).