Look at the plot that resulted in the three-way tie in the underwater Defense battle. Clearly the universe of this fic allows absurdly complex plots to work when the plotters are of a high enough level.

Quirrel himself noted that plans that contingent on uncontrollable events tend to fail. Dumbledore wasn't counting on that plan working, as he said, "That's why it's important to have more than one plot going at once." The plan that resulted in the three way tie succeeded largely due to luck.

Look at the plot that resulted in the three-way tie in the underwater Defense battle.

It only looks complicated. Yes, it was not extremely probable that Blaise would be amongst the last few standing. But very quickly all teams raced towards an even score in a fairly predictable self-organizing way. So there weren't really a lot of moving parts there, just one complicated system with fairly predictable behavior, and some complicated-looking plotting to get Blaise's cooperation, and just a bit of luck that Blaise didn't get knocked out earlier (helped by Blaise being mostly interested in survival instead of racking up points)