have I completely missunderstood all previous teaching that max range IS achieved at best L/D speed?
Indeed, for a jet yes, but your understanding is correct for a prop. As Old Smokey says, for a jet max range is generally faster than l/d speed. I would add that min drag reduces towards l/d with altitude (at max service ceiling they are just about coincident). L/d speed is pretty much the same as clean speed (or flap maneuvering speed with flap). L/d speed is best endurance speed and what you want to fly in a hold.
However, it would seem to me that If really desperate to save fuel, then use l/d speed for descent.
Rule of thumb I've used is increase TAS by 40% of headwind, decrease by 40% of tailwind. Min speed is max endurance. Max speed is where the knee in the fuel flow / TAS curve is (you will likely have to estimate it).
I had not heard of that one. My guess was that given that a typical drag curve is very flat at around min drag speed for a jet, I assumed that one would want to increase speed into a headwind by much more than one would want to reduce it with a tail?