I think you may be trying to fix something that we don't yet know is broken.
(or "badly" broken, anyway)
I had a look at a bunch of rear view images of your car, and there's a not insignificant amount of plan taper in the greenhouse, and quite a gentle transition (very generous radius) in the C-pillar from "side" to "rear" surfaces. All of which suggests to me the risk of vortex formation on this car is probably not big.
I'm away from home, so I don't have my copy of Hucho handy, but he laid out a chart comparing various rear hatch/fastback angles and their effect on Cd that illustrated the problem of vortex formation. But my foggy memory tells me your hatch isn't in the "danger zone".
If I were you, I'd start at the start: tuft testing your rear glass. It doesn't take 10 minutes to set up (and you can even do it with 8 inches of snow on the road
).