[PaleMelanesian]

Unless the transmission free-wheels when you let off the gas, and drops to idle rpm, you're wasting your time trying P&G in gear. DWL / steady speed is better. The momentum lost due to engine braking in gear is more than the gains. If it does free-wheel, then your job is easy. GM transmissions are good about this. Honda's are bad - they hold the gear and engine-brake your momentum into oblivion.

You will NOT damage the transmission by shifting it into neutral with the engine running. EOC might, but check your manual for flat-towing instructions.

Also, the higher your speeds, the smaller the benefit you'll see from P&G. 70 is too fast. Consider that the aerodynamic drag increases with the square of your speed. Your glides will be more like "plummets", instead.