I am too. I mixed up the two little transborder flights in my post. It's fixed now.

OP has a ticket, at about $2000 RT TPAC in business class, for YYZ-DTW-PVG and return, ALL DL marketed and DL operated for the special price.

The contemplated purchase for the positioning flight would be LGA-YYZ operated by Westjet but sold as a DL codeshare. I assume it would leave from a different area of LGA from the terminals DL use, probably the central terminal, so that if there's an obvious delay, there would be no opportunity for the OP's two tickets to be changed to LGA-DTW-PVG, all on DL of course.