That was what I thought Nokia should have done in the first place when they made the camera grip. Not for lens adapters but for screw on ND filters and polarizers.
But then when I was thinking how it should have been designed, I realized why they didn't do it. The camera flash will be inside the lens or filters if the thread is on that case's opening. I still won't mind it since I never use flash. But I guess Nokia had to avoid returns on customers not understanding that and blame it on the case.
On the note on your attachment lens producing unwanted weird effects, it's not your attachment design, it's the lens being attached. Digital sensors do not take stray lights entering at non perpendicular well, unlike film ( which is why it had been a challenge to do full frame with similar sized 35mm lens and Contax Zeiss had to go to medium format lens to do full frame back in the days, and we had to have smaller sensor for 35mm sized lens ). So lens are designed specifically for certain sensors, how those sensors receive light, and work on their advantage, while minimizing defects. Almost all attachment lens has no capacity to go through that design and engineering process. Anyways, just pointing out it's the lens attached, it's curvature, it's coating, etc. and not your idea/design.