I have a Lumia 535 and it works just fine, bought it over a year ago (I probably dropped it on the ground at least 50 times since then, no cracks at all). I still haven't experienced any problems with it. I always hear a lot of praise with the 640, so maybe you got a faulty device, that could explain some of the problems you've been having.
Also, did you upgrade your phone to W10? I'm asking because a lot of your complaints (Like the terrible settings, no flashlight button, etc..) were fixed with the new OS, just saying...








