Flight rules are likely a bit different in a large open country dotted with urban/suburban areas like the US but under the rules you cite a single engine aircraft couldn't operate in and around a large city. I've learned that in a large city like Los Angeles, small private aircraft have to follow very strict protocols because the airspace is teaming with large commercial aircraft that have priority. As far as those rules are concerned however, it's my understanding that they are immediately waived during emergencies with life and limb becoming the priority. My GF tells an interesting story about experiencing engine trouble in a rented single engine Cessna several miles out over the ocean from LA and they cleared the airspace for miles around and guided her in for a landing on a main runway at LAX. Turns out the plane had a history of carburetor troubles.