You can't automatically base his Wind Scar attack in the movie as weak in power because he didn't destroy a mountain with it in that scene. For all we know it wasn't an attack of lesser ability in that scene, but an attack with more
control. As for Sesshomaru destroying a mountain with it. Well, the attack was clearly powerful, but was it really controlled? focused? Doesn't seem so to me.
Also, they all know the sword could kill 1000 demons in one sweep, but how did they know that was the truth? I think Inupapa killed quite a few demons with that sword when he had it. A lot more than a few low level human soldiers that really didn't require a full blast of the Tessaiga to defeat.
There is nothing in the manga I believe that indicates that Inupapa did not master the backlash wave. (Somebody quote me a chapter if they say somewhere that the dad never mastered it. I don't remember reading that bit.) The only thing Inuyasha did that his father did not was add to the sword's abilities. However of the abilities that were already there when Inuyasha inherited the sword, I think it's a safer assumption that his father could use those abilities at full strength and greater level than his son.
At the beginning of the manga:
Inupapa > Sesshomaru > Inuyasha
By the end of the manga:
Sesshomaru > Inupapa > Inuyasha
That just IMHO. Discuss!