Once an effect has been placed on the Chain, it'll resolve unless negated. Missing of timing is used for optional effects that cannot be placed on the Chain due to the Game State not corresponding to its trigger requirement.
In your example, the current Game State is the normal summon of Debris Dragon, which correspond to the trigger effect of Debris Dragon. There is no missing of timing.
Super Polymerization's last effect means that no other effect can be added to the current chain, which is to say that Super Polymerization will be the last effect in that Chain. It does not affect any effects that are already on the Chain.