1. Centipede waking up from wrong side of bed doesn't matter in this problem. Since the centipede gets the slippers randomly, the problem is asking about the worst case which results in the most time taken.
2. If the centipede finishes all the left slippers, i.e. 40 of them , there are still a-40 left slippers present under the bed. The problem doesn't even mention that after wearing 40 left slippers, these a-40 left slippers will also be thrown taking 2 seconds. Throwing is only mentioned for right slippers.