Compelling story or not, if the Mavs lose game 6 they're done, history, end of story.
Just last season, the Lakers came home facing the same situation the Heat face now, down 3-2 with the story being that the "old demons", i.e. lack of toughness, were still haunting the team. They won the next two and "re-wrote" the script. Now I'm certainly not comparing the Mavs and Heat to last season's Lakers. The point I'm making is that so many people's opinions seem to get shaped by what they most recently see.
The Heat really accomplished the goal they needed to accomplish in game 3. That win ensured that the championship would be decided on their home floor. If I were the Heat I would be thinking, 'the Mavs won two important games in a row at home, so can we'. If that happens then the story will be how Wade overcame injury to will the Heat to another championship or that LeBron finally showed up in the 4th quarter.
On the other hand, if the Heat really do have feet of clay, we will find out this Sunday. And if that happens, I don't want to hear squat from Lebron fans claiming he is better than Kobe. Kobe closes the deal. He demands the ball in the 4th quarters of the big games. In other years when LeBron lost in the playoffs, he, his apologists, and the media, blamed "un-talented" teammates. How will they do that this time? The "King" had better damn well show his aristocratic bloodlines Sunday or he will face the unfiltered wrath of NBA fandom for the forseeable future.
As for Dirk, if he loses another one, especially after being up 3-2, then he will remain a 'not ready for prime time player'.
All that said, the Heat are the more talented team and they have HCA and the last games of the Finals on their home floor. They should win it. But unlike last season when the Celts didn't even show up for game 6, the Mavs, I believe, plan on making an appearance.