Using a thin case/holster would most likely address #2 and #3. Depending on how unobtrusive you want it, you may have to search around a bit. Many are padded a bit, which adds some bulk.

Or maybe get the holster and belt carry it, although some people may not want to look like they're carrying a folding knife on their belt.

Oh, hehe, I forgot my own personal carry method. A little while back, I developed a hole in my front pocket that I didn't notice and I lost my Fenix and Space Pen through that little hole. So I bought a small pouch (you could call it a coin pounch) that zips shut. I put my Fisher Space Pen, Fenix 1AAA light, and Leatherman Micra in there. Keeps everything together and a bonus is less wear and tear on my pockets! I think that should keep a flashlight from twisting in your pocket.

I'm surprised that the head on the Quark Mini can fall off simply from being in the off position. Or do you mean the head continues twisting in the pocket and THEN falls off? I have never had a twist-head light twist apart or turn on in my pocket before. Maybe by wiping off as much of the lube as possible from the threads, you can avoid that fate.

Twist switches do have their advantages. They are smaller, simpler and less prone to failure than click switches. In my own personal experience, the main problem I've had just once or twice is unintentionally over-tightening the head on lights that don't use spring contacts (these are typically compact 1AAA or 1AA lights). Sort of crushed the electronics in the head, although the lights still functioned.