Unfortunately, the connector that locks the camera to the harness (it works well and released from the harness easily and quickly) uses the tripod mounting screw hole, and must be unscrewed to be able to mount it to a tripod. Not a very difficult task, but not as convenient as I'd prefer.
I have the same problem with the right "clip" on mine. I gently spread the tines on the upper part of the "clip" each time I use it and it holds better. My primary camera is a Nikon D7000 with an 18-135mm lens. I carry my Nikon D80 with a 10-24mm lens in the second holster. I would like to keep my 70-300 lens on the D80, but lens creep makes that out of the question. The cameras do seem very heavy, but things stay together now. I believe part of the problem is the way the clip is attached at an angle which seems to put more stress on one side of the "clip." I plan to replace the "clips" with heavier spares I have if the problem continues to worsen. I really like the carrier and my only complaint is with the small "clips."
Yes - it stays close to the chest. Only problem I had with this is the button that holds the camera in place gets stuck. For your car photos it's not a problem. But I'm a birdwatcher and bought it to take pictures of birds because it has a really good zoom. You need to get the camera out quickly to catch the bird and the camera would get stuck . . . . . and the bird would fly away before I got the pic.