1. Yes. As mentioned, it helps keep down spray and debris for following riders. My GF regularly drafts me and appreciates the fender and flap. And, a rear flaps looks nicer.
2. Yes. I use neatsfoot oil on my saddle and flaps, as it soaks right in and works brilliantly. It helps to protect and keep them pliable. I've never used Proofride.
3. Yes. I used a drill and took my time. I put a piece of masking tape across the fender, to help with marking hole locations.
Also, ditch the included nuts. Go to the hardware store and buy nyloc nuts. Install them with threadlocker. I noticed the included nuts loosening, with every ride, and kept tightening them. Eventually, the rear flap fell off, along a busy highway. I didn't notice until I stopped for a break. I had to backtrack to find it. Fortunately, it was only a few miles back.