It is something that needs to be learned, if not before the engine fails, then very quickly after. It is this skill that will get the pilot back onto the ground. A power plane without a working engine is nothing more than a glider with poor performance.
The speed for straight and level L/D is the speed for best climb. If in doubt fly the speed for best climb at MTOW. Also most operating manuals give a "Max. Glide Speed" (i.e. the speed to be flown to achieve max. distance) which may also be the engine out landing speed. At 45 deg multiply this speed by root(2) (i.e. approx 1.5). The polar varies as the square root of wind loading and so is not so sensitive to weight as you may think.
What you seem to be saying in your post is that pilot training is inadequate. Fortunately, as the video shows, this is not universal.