Did cruiser gunfire force scout planes overboard?

How often did main-battery blast force cruisers to sacrifice aircraft waiting on their catapults?

Very rarely, because aircraft were normally stowed or launched before engagement; even in combat they could sometimes be launched between salvos.

Scout aircraft were normally kept in the hangar or launched before battle for reconnaissance and spotting. That routine made it unlikely that an aircraft would be sitting exposed on its catapult when the main battery suddenly opened fire. Only an exceptionally unlucky pre-radar surprise at dawn would create the feared combination of a prepared aircraft and an enemy already inside engagement range. British cruisers at the River Plate could even launch aircraft between salvos despite having catapults constrained within the ship's gun arcs.