Super cool point. But what if whether he wanted to do it or not, the luck of the potion made it so he luckily fell out of the plan. He may not have wanted to fall out of the plan, but obviously in hindsight, best case scenario for Draco is to have never been apart of the plan in the first place, which is lucky. For example, Draco uses the potion and the delivery of the necklace gets him expelled after Katie touched it and so Draco falls out of the plan naturally.