First, Welcome to EM...
The assumption of 1.0GPH may be a little high, which may effect the cascade of theories. Re-run the calcs at 0.5GPH and you may find a better result for coasting (20 mph becomes 40 MPG, 10=20, 5=10). Or 9999 mpg with the engine off
<Disregard that last statement, unless you are aware of the risks, practiced on a closed course with a professional driver, etc...>
Someone else can help with P&G, but the slow-to-light in neutral seems to be a tried and true method for a variety of vehicles. If I come in too hot, I downshift to force the injectors into "DFCO". The worst scenario is to start from a standing stop.
Another method (not as popular) is to keep up the speed and stop quickly. Your theories would apply here. 100 mpg, 100 mpg, 100 mpg, STOP!
RH77