Would the fleet demonstration bypass Congress and start a war?

Would sending the fleet on this demonstration amount to a preemptive war without congressional approval?

No. The fleet is demonstrating rather than attacking, while Japan still declares war and opens hostilities against American forces.

The fleet sails to demonstrate resolve, not to open fire before Congress acts. The movement itself therefore does not become an unauthorized preemptive war. Japan's declaration and attacks still provide the belligerent acts that bring the United States into the conflict. The constitutional concern would change only if American forces initiated hostilities, which this scenario does not require.