Why 18-inch guns alone could not save Singapore

Would fully traversing 18-inch guns have prevented Japan from taking Singapore?

They would help, but they would not by themselves fix the command failures, weak defensive preparation, and loss of the reservoir.

All-around 18-inch guns with suitable ammunition would strengthen Singapore's defenses, but they would not provide a decisive cure. The deeper problem was command: Arthur Percival struggled under pressure, the defense failed to stop the crossing, and the reservoir was not secured. Heavy guns could punish targets without reorganizing the retreat or protecting the water supply. Their value still depended on the institutional and operational conditions needed to exploit them.