Assuming the number of flying cars is equal to or lesser than the number of regular cars, none of your scenarios is likely, especially the concept of the airways getting so dense that we run out of airspace for flying cars. Yes, fuel is finite, but it's not that big of an issue when proper planning (and autopilot telling you when you're going to be getting out of the air to refuel) is involved. Parking does indeed require ground space, but if you can reach the incredibly-hypothetical scenario where everyone, or at least most people, flies cars instead of drives them, the amount of roadway you need to access parking space reduces dramatically, freeing up space formerly taken up by, say, freeways and eight-lane roads, to be used as parking space. Dense urban areas, it turns out, have more large roads and MUCH more freeway (per unit area) than "regular" areas, so you could save a lot of space if you could get a sufficient number of people into an as-yet hypothetical flying car capable of extremely short-runway operations (or VTOL. VTOL's cool too) on a budget acceptable to an average person (so about the same price as a non-flying car is right now).
Yeah, fine, it's not likely. But it'd be cool and useful. Oh, right. This is a derail. Sorry.