Replacing Spark 720 2015 derailleur + hanger using markings Broke my rear derailleur a week ago along with the hanger.

On my Spark 720 year 2015, the old hanger said 142-RWS12 IDS-SL so I ordered a new one from eBay:
https://www.ebay.com/itm/SCOTT-23528...-/291980522554

Bought a new Shimano XT RD-M786 derailleur from a local shop. Both say "Shimano RD-M786 VLA" in the recess.

The parts fit together but it's clear that the reach of the derailleur will never reach the largest sprocket no matter how the adjustment screws are set, so my suspicion is that I've got the wrong hanger for the frame+derailleur combination, despite the markings being the same as the old equipment. The derailleur can't reach as close to the wheel as needed.

Which is the correct hanger?
Derailleur hangers are typically made from a flat plate of alloy, and are not bent to position the derailleur in or outboard.

I'm guessing, but if you have the correct hanger I suspect you have it mounted on the wrong side of the frame. Googling the serial number for images of the hanger I see the hanger mounts on the inside of the frame and there is a matching plate that mounts on the outside.

The google images I found seem to match online images of the Spark. If your replacement hanger looks like those you probably have the correct part.