Even if LIVEATC had complete coverage of a flight, you might not be able to follow it since most Center feeders scan several frequencies on a feed. The scanner may be stopped on another frequency when your flight calls in to ATC and you will miss the call.
That's correct. ATC sectorization is complex, and can change even during a 24-hour period. So even if we had coverage everywhere (as noted, we don't) it would still be difficult. Still, we are working on making tracking a little easier than it is now. But it will still require a little knowledge on the user's part. It will never be easy, because it is inherently a difficult problem to solve.