Questions regarding descent (specifically in a 172R)?

You have gotten some good answers to your very good questions. I noticed on thing in your question that made me think, and agree with the responder that said look out of the airplane, because the visual feedback is what will inform your approach most accurately. However, the item about reducing power to 1500 rpm, may be destabilizing your approach a bit. If you pull the power back to start the decent and add flaps or whatever you do to establish a fairly smooth rate of decent and the air speed and VSI don't give you what you want, use that lever in your right hand (throttle) to adjust the rate so that you arrive. The tach is not the bible, 1500 rpm is not the goal. Landing in a nicely controlled safe manner is the goal. Once you have set the RPM you will be close, but the throttle is in your hand for a reason; it is your means of adjusting your rate of decent.