let us know how it goes. I tried this set but the polarity of the magnet doesn't work with the N5.

Yes.

I've used This pair of lenses and This one. I don't think it is possible to use lenses without blocking the flash at least a little. The adhesive rings work great, I put one on my front facing camera for my nexus 7 for wide-angle video chat, and one on top of my Black Dbrand leather Nexus 4 skin. I dont know about the Ringke Slim since I don't own one.

The first lens set I linked was the best but cost more. I mostly liked it because the wide angle didn't vignette. The second lens I linked does get some unavoidable vignette on the corners which only shows up in photos. Video crops the corners out so you don't see it. You can zoom a little to get rid of vignette in photos at a small loss in quality. Let me know if you have any other questions.