<[1] How is the fuel economy on this
flying-cars? Is it advantageous compared to wheeled autos?

There have been so few flying cars made that it's hard to give a good answer. The Aerocar was a product of the 1950s when fuel economy wasn't a major factor. From what I've seen online (see link below), the Molt Taylor Aerocar got about 18 MPG on the ground and about 13 MPG in the air.

The new flying car design that the owner of <a href=http://www.aerocar.com is proposing will use a Lotus sports car for the auto portion. According to this source, he's going to replace the Lotus's engine with a lighter 3 cylinder engine and transmission from a Chevy Sprint. As for when it's flying, I don't know how many gallons per hour it'll burn at cruise speed.

On average, most light planes get relatively poor gas mileage compared to most cars. I typically cruise in my Cessna 172 at 105 knots (~120 MPH) burning about 6.8 gallons per hour, getting about 17.75 MPG. I know of other, more efficient planes that can cruise much faster with fuel burns that aren't much higher than mine, so they get considerably better mileage.

[2] What happens when terrorists or criminals get behind the 'wheels' or fly-by-wire controls of such flying-cars?

The same thing that happens today. Drug runners will still attempt to smuggle using planes but I doubt they'll want to use flying cars because they won't be able to carry as big a load or fly as fast as other planes. Most light planes really pose little terrorism risk because they can't carry enough of a load fast enough to do serious damage. Do you remember that Florida kid who stole a Cessna 172 and crashed it into a building (around November of 2001)? He only damaged a single office. Had he packed the plane with explosives, he could've done more damage but not a lot. Larger light planes (like a Piper Navaho or Cessna 310) fly a lot faster with a heavier load, so they could do more damage, but still, that's nothing compared to what an airliner can do.

3] (This question would be appropriate to your geography, Larry) What happens when such flying-cars face obstacles like a range of 14,000 ft-high mountains? Would pressurized cabins be required for such flying-cars in such circumstances? [Aviation-medicine research indicates a hazardous drop in physical & sensory responsiveness above 12,000ft elevation]

Actually, flying cars like the Aerocar have a big advantage over other planes in this regard. Molt Taylor designed the Aerocar so that the wings and tail could be detached and towed behind the car. In the event of bad weather or severe terrain, you could land, detach the wings and tail, and drive off in about 15 minutes. Once clear of the bad weather or terrain, find the nearest airport, reattach the wings and tail, and you could be flying again in about 15 minutes. Damned cleaver.

Only a small percentage of light planes are pressurized due to the expense. If I fly above 12,500 feet for more than 30 minutes, I'm required to go on oxygen. I've had my Cessna over 12,000 feet a few times without any problems, but I've lived almost 7,000 feet above sea level for many years so my body has acclimated. Every year, we have marathons here in Colorado where runners do things like the Pikes Peak Marathon (14,110 feet tall) without oxygen. Of course, I don't such things because I'm convinced that running causes brain damage. The most extreme case of altitude performance I can recall was back in 1978 when a couple of exceptional mountain climbers made the summit of Mount Everest (over 30,000 feet) without using oxygen. Those two guys were studs.

While pressurization is very comfortable compared to an O2 system, it isn't absolutely required until you get above about 40,000 feet. Above that altitude, simply breathing pressurized oxygen isn't enough - you have to have pressure on your body. This is done by wearing a pressure suit or by using a pressurized cabin.

Here's a link to the Aerocar Model III. As you can see, it's a lot nicer looking than the Model I and quite a bit faster, too.