I’m just unclear where you’re finding such a large difference in depreciation.

A 2018 Stinger GTs 36m residual is 54% and the C43 is only 58%. That’s really not that much and I’ll put money on it they depreciate almost the same % in 4-5yrs. KMF is simply basing residual values on current Kia cars, not a sought after unique model like the Stinger...
The real factor on why Stingers lease payments are worse is the dramatically higher money factors(APR%).

Benz is willing to take a hit on financing profits by consistently selling higher quantities over many years, while Kia is trying to offset 1st year losses with some long term profits. It’s just each manufacturer adjusting their balance sheets and all these factors will change in a few years...
At least for now it seems like the price advantage of buying a Stinger in Canada makes it a complete steal! While leasing(or lease with immediate buyout) in the USA has the most advantages.
Hope this helps dispel some of your depreciation concerns, and hope a better deal comes along so you too can enjoy what a unique experience it is to be a Stinger owner.