Because the Coral itself has the attributes to change and disrupt the natural laws of physics, it's due to the Coral itself that entire Limit is a threat.
Basically, the Coral had grown to such an extent that its own existence was a threat to itself, so it had to force its entire existence into sleep. That why the Great Wall is an area that is not bound by normal physical laws. Thousands of years ago, a huge meteorite fell and awoke the Coral, it managed to put itself back to sleep, but the area that was hit remained in a state of anomaly.
As for the poison, Dewey's complete plan would have originally been able to completely neutralize the Coral with minimal casualties, but if it couldn't happen, then he was prepared to destroy it at the cost of everything else.