So Bismuth insists that her wrist mounted pile driver would've given the Rebel Gems a chance, but why and how would the Breaking Point even achieve that?

We don't know enough about the war, how it was fought, when, what numbers, what kind of battles there were.

And also what is the tactical advantage of shattering over poofing? Isn't there plenty of ways to OHKO poof a gem than use a weapon that was a cumbersome wield and maximum range of less than half a metre?