Your making it sound like it's perfectly fine that Kirito pulled dual wielding outa nowhere and soloed a boss that shouldn't even be soloable, but it's unthinkable that Heathcliff would have access to or should I say, the smarts to be able to take advantage of the equipment required to play the game and use an ability that possibly anyone could use. Just as batou01 said, if your going to consider Heathcliffs ability cheating, you might as well consider Kirito's cheating as well since as far as we know, he's the only one who can use it. Hell, I'm in agreement with batau01 about who Heathcliff might actually be, it just seems to make sense.
Also, it's not that the game rules are "mostly fair" it's that they can't break the code to allow certain things to happen, for instance, the killing of players in safe zones, it's just not possible. So Heathcliff having an ability to increase his speed to the point that time around him slows to a crawl wouldn't be considered cheating, just an ability that was already programmed into the game that he and possibly others have access to so long as they can figure out how to obtain it. This would also apply to Kirito with his dual wielding. Granted, something later in the show could disprove all this, but as of right now this is how I see it.
Another thing, how are pk guilds considered unfair? It's part of any mmo you come across. The point of an mmo is to allow you to play the game however you want and if you want to run around killing other players for pvp fame then you should be allowed to do it. The fact that those who get pked in this world actually die just makes it more of a taboo thing to do. It'd be more unfair to not be able attack other players, hell it would be a step back in game design.