You are correct, drag reduction while maintaining usability is the goal.
Correct, air can stay attached up to about 30 degrees, which is great for keeping the rear windshield clean on lots of fastback cars, but terrible for their Cd!
You will never have 100% of the air that went over the top fill in the wake, no matter what template you use. The air is at a higher density and velocity than the air in the wake and wants to move toward that area (or any lower velocity / lower density area) through the path of least resistance. If that happens to be up, down, left, or right, that's where it will move. Much will fill in the wake area, and that's what we are trying to maximize.
Yes and no...
Yes, if you plan on taking your vehicle all the way out, following the template to completion, then yes, follow the template.
No, if you plan on cutting the vehicle anywhere short of the full template, use about a 12 degree angle. Hucho (and many others) have shown that for a truncated body, something like 10 or 12 degrees is the optimal for drag reduction. Granted, the tests done to generate these drag profile curves were done without making super smooth curves (like the template has), but they have been proven time and again to be about as good as you are going to get.
Look at these graphs, and ask yourself, where would the aerodynamic streamlining template have me be? Between 3.5 and 7.5 degrees? Following the logic above, this would be optimal, but it isn't... something more like 10 or 12 degrees wins the day.
Not to mention having your super long boat tail smash into things when turning!
I checked out your other post (reminding me that velocity was involved... tisk tisk on me!) and I will be commenting once I am done reading.