First of all it's all hypothetical and it doesn't matter when Mayweather first fought at 140.
The fact is he has fought at 140 at some point. Secondly this is the fight that Mayweather needed to have to shut up all of his detractors.
Forget about the Showtime/HBO thing, if he really wanted it he could have got it.
Mayweather was always a bigger name in the US than Tszyu and something would have been worked out. Tszyu was simply to dangerous an opponent back then, his 1999-2001 form is outstanding and it would be difficult to see how Mayweather would not get caught at some point during the fight.
He would need all of his defensive skills to avoid Tzsyu who banged just as hard with his left as his right.
We only saw the right hand so much because of his dominance over Southpaws.
It would go the distance because of Mayweathers skills but a younger fit Tszyu back then was awesome.
Hatton got him when he was old and struggling with weight and injury.