Why a 5.25-inch cruiser mount was not a quick substitution

Would a between-deck 5.25-inch dual-purpose mount have been a better cruiser option if the Royal Navy accepted the design compromises?

No; the mount would require enough redesign work to delay the ships by roughly two and a half to three years.

The answer treats the weapon as part of the ship rather than an interchangeable component. A between-deck 5.25-inch mounting would demand major structural and systems work, delaying delivery for years. An apparently superior weapon can therefore be a poor procurement choice when integration time matters.