Heres some facts(modern(jap efi,afm/map,o2,tps car after ~86 or so)):
When you're coasting and press the clutch pedal, you're forcing the engine to idle, and thus it requires fuel injection to stop the engine from stalling. It may rely on certain information to take care of this automated procedure such as tps_idle=true (close throttle switch) and thus will switch to a fuel map to maintain engine rpm/idle motor control at your set value.
However, you're wearing the shit out of the thrust bearing on the gearbox input shaft, and probably glazing your clutch/flywheel/pressure plate also.
If you put the car into neutral, the input shaft would remain connected - you're driving even more equipment and thus requrie even more fuel to maintain idle - completely independant of speed.(It's not much, but its more than coasting whilst engaged at the highest gear possible)
Exactly the same, except it will probably inject a certain(small) amount of fuel to keep the rotation of the engine constant, due to the jerky pulses applied by the torque converter when being driven from the opposite end(turn a car off in gear, in an auto and you will probably feel it)
Both the manual and auto car may inject a small amount of fuel whilst coasting. but its teeny tiny amounts!
Theres only one way to maximise fuel economy. catch the bus. but if you want it to hurt less, drive like a granny and abuse that 5th gear