So, we need to postulate:

1. Polyjuice potion made a month ago, by first years, for no reason they could know in advance
2. A student who is an excellent enough actor to mimic Neville on the fly
3. A student who is also skilled enough to learn the Breaker move (on top of his usual classwork and, of course, the Polyjuice potion)
4. A student skilled enough at tactics to take on Weasley and win (see 1-3)
5. A student who is somehow skilled enough for 1-4, but not as useful as Neville (so that becoming Neville's dead decoy is a net win)

I dunno, all of that seems like an asspull to me. And to what effect? Neville could've just swam off to 'play assassin' without all this. 'chaos' doesn't seem like a sufficient explanation.