I really suggest you read my entire post.
"....they could be using Hydrogen, which is far more compressed and easy to carry large amounts of compared to consumption..." No, that's wrong. Although hydrogen has a low mass, it is one of the least energy dense fuels, even in a liquid state (liquifying hydrogen requires immense amounts of energy, while plausible when you have fusion power). Actually the hydrogen fuel is one of the things in Halo I find the least plausible.
Here's a cite (you can wiki this too if you like):
"Hydrogen has more energy per unit mass than other
fuels (61,100 BTUs per pound versus 20,900 BTUs per pound of gasoline).
The problem with hydrogen is that it is much less dense (pounds per
gallon) than other fuels. A gallon of gasoline has a mass of 6.0 pounds, the same
gallon of liquid hydrogen only has a mass of 0.567 pounds or only 9.45% of the
mass of gasoline. Therefore one
gallon of gasoline yields 125,400 BTUs of energy while a gallon of liquid
hydrogen yields only 34,643 BTUs or 27.6% of the energy in a gallon of gasoline.
The Space Shuttle uses hydrogen as a fuel, because its mass is low, and
the fuel is carried in an external fuel tank that is jettisoned during lift off.
Automobiles can not have external fuel tanks that are discarded, and the
energy per unit volume is used to determine a fuel’s energy density in
automobiles. Compressed gaseous
hydrogen is even less dense than liquid hydrogen. At 5,000 psi of pressure gaseous hydrogen only has a density
of 0.25 pounds per gallon or one twenty fourth the density of gasoline.
Gasoline and diesel are far superior fuels to hydrogen in this regard." http://www.dalefield.com/slspartners/hydrogen_fm.html
"...Its very possible to have the same propulsion in a turbo jet..." Yes, and you can use energetic radioactive particles from a nuclear reactor to create a jet--it's been done before. http://en.wikipedia.org/wiki/Project_Pluto
If only you could shield the reactor...
"Have you seen how small the pelican jets are?" The engine's themselves look like they are about 15 feet long http://halo.wikia.com/wiki/Image:PelicanII-Sil.gif the little exhaust ports are small, but that's not at all remarkable.
"Where does the propellant come from and how does it remain in the air for such a long time?" I'd like to think that it uses some form of miniaturized nuclear power, fission or fusion, to turn turbines combined with some sort of fuel (even hydrogen if you like). The effect achieved is a jet engine that, with much stronger future metal alloys, can achieve very high compression but also has the benefits of being able to fly out of atmosphere because it has a contained fuel source instead of needing to breathe atmospheric air.
"The engines on the pelican could very easily be adapted to smaller, more nimble aircraft and move it at a much higher speed due to its loss in wieght and cargo." The Sparrowhawk probably does use jet engines like the pelican's, but as you said they are there to make it go fast.
"Propellers are one of THE most vulnerable types of propulsion, because aircraft that rely on such propulsion have no other option than to fall to the earth if it is lost." The Sparrowhawk doesn't rely on just hoverfans, it also has wings and jets for horizontal flight.
"Helicopters may be maneuverable, but there is a reason they are coupled with ground troops, to get the fire away from them. They aren't assault vehicles, but support craft." If you consider the role of a bomber to be support, or artillery's role as to provide support, then yes--attack helicopters are in a "support" role. "The only existing assault aircraft with propellers used for main maneuverability are Helicopters, which as I said are support craft\ and are only truly effective when coupled with the assistance of Air and Land support." No unit is effective by itself. What about the AC-130 Specter (look it up on youtube for an awesome display of firepower)? Why does the cutting-edge F-35 joint strike fighter have a lift assist fan if they are so crappy?
"You know why Helicopters rarely get shot down? Because the Air Force gains air superiority before they fly, and ground troops usually take care of the majority of enemy infantry and armor. Helicopters are purely assisting aircraft." That's a matter of opinion, in vietnam our helicopters were effective even without air cover, in afghanistan the Soviet Hind was so succesful that the muhajedeen said that "We do not fear the Soviets. We fear their helicopters."
You keep talking about improvements in technology, but you haven't mentioned improvements in fibers and metals. The fact that there is a giant space elevator in Halo 2 means that they've made a material with several times the tensile strength of kevlar, I'm betting this technology existed 30 years before Halo 2.
Also, the Sparrowhawk isn't really even a helicopter, hover fans are just the part of it that allow for it to hover in place efficiently.