Item 1. Coasting is manageable but risky with the engine off on non-hybrids.

Item 2. I am fairly sure I am correct, but if you know better, I would like to be corrected. The portion of engine power to increase vehicle speed is entirely saved and usable as increased vehicle kinetic energy. The increased kinetic energy shows up as larger MPG values (on uncompensated meters) as you later coast back to the original speed, and do not use as much fuel to do so. I have built MPG meters that compensate. An uncompensated meter just sees larger fuel consumption and registers lower MPG during acceleration. It can be misleading.

Shell Oil used to have MPG competitions. The common practice was to shave the tread off the tires and inflate them to 100 psi or so. Then accelerate at max throttle to a high speed, then shut off the engine and coast to the low speed and restart. The idea was to decrease rolling resistance, and use the engine, which is after all a heat engine, at an efficient power, and then shut it off. It produced MPG like those you get on P&G. If light throttle operation were efficient, then P&G would not make higher MPG values.