I haven't experienced this issue, but I sympathize and I know where you are coming from when you state that once the check engine light comes on, you have no way of knowing if it's about the issue they told you not to worry about, or some new, unrelated, issue that maybe you should be concerned about. I've been in that situation before.
While your situation is clearly annoying, concerning, and as you pointed out, problematic if you wanted to sell the car soon (before the issue is resolved), I'm not sure that I agree with FlNiro
that it rises to the level of a lemon law kind of issue. But I can imagine how it could. I guess I see both sides of the fence on this.
Have you tried to run the onboard diagnostics when the engine light comes on? If so, what does it tell you? Perhaps that could give you a little peace of mind if it can reassure you that the only issue is the one that they told you not to worry about (assuming you believe that guidance).
In other cars where I've experienced a check engine light kind of issue, I know that some of those issues will self-resolve and the check engine light will turn off automatically if the car goes through one or two full cycles with no negative input from the sensor that originally turned the light on. I don't recall the specific details of a "full cycle" but I vaguely recall that it has to do with letting the car cool off (as it would if you parked it over night) and then starting it and driving it and letting it reach normal operating temperature. So while you're in the situation of being forced to put up with this, perhaps if the check engine light comes on because you shut down with climate control still enabled, you can get it to turn off by being more diligent in turning it off on the following one or two days, before you shut the car off. Not that you should have to do this, especially for a car under warranty, but just that it might make the current situation a bit more tolerable if this works, until such time as your dealer can provide a better solution.
If I was confronted with what you have described, I think I would begin by sending a written complaint to Kia Corporate, via registered mail. Hopefully that will get their attention, but failing that, it will put you in a better position to bring a lemon law kind of complaint if this escalates to that kind of situation.
Please let us know how this works out for you.