1) it makes a good marketing case. Audiophile headphone companies don't usually aim to make their headphones sound good above 15khz, but if the headphone somehow ends up producing those sounds, why not list it? But yes, anything over 10khz is mostly useless. As for sub bass frequencies, they're hard to hear, but really easy to feel. Unfortunately headphones are not the correct medium if sub bass is important. Only subwoofers can do them any real justice.
2)Linearity within the audible range (20hz - 10khz or higher) is better than end to end extension.
3) technically yes. However, even though companies list their headphones as being able to go that deep,they don't mention that the sound is reduced by a few decibels at that frequency.
Now, to introduce you to a better way of looking at frequency response.... enter the frequency graphs!
Just for fun, i included some really well balanced world-class headphones (lcd2, hd800), a bang/buck bass heavy headphone (vmoda m100), and a lower tier grado headphone (i don't usually like grados, but just to show you what bright and sub-bass deficient headphones look like).
The higher the line on the left side is, the heavier the bass, the higher the line on the left is (however, don't bother looking past 10khz), the brighter the headphone are. The reason i don't usually like grados is because the grado house sound usually lacks subbass, and are bright. The lcd2 and hd800 are balanced, but as you can see on the graph, the hd800 will be noticably brighter since it has more treble.
Another site that has many graphs is innerfidelity.
While frequency graphs can't tell you how a headphone will exactly sound like, the more headphones you actually hear while being able to look at their graph, the better you will get at guessing a headphone's overall sound signature just by looking at the graph.