I admit a gap in my knowlege of naval tactics during Lepanto-era. I'm betting that the heavy-caliber cannon fire was directed pretty much in the forward direction, because the galley was strongest along its keel, and therefore gunfire was used as a prelude to ramming. Firing guns at any other angle would likely induce a destabilizing roll. Guns that were directed outward along the beams of a galleyas would probably be used to discourage ramming or to clear the decks of an opponent prior to boarding.
I do know that during the later period of the Spanish Armada, the guns of the Spanish galleons were basically impossible to reload -- so naval gunfire was basically a one-shot, or one salvo affair. Only the English had a compact gun carriage that would permit reloading, but this was still some years off. Also, ships of that later period were generally not sunk, they were more commonly boarded & captured. But with the less robust construction of galleys (relative to galleons, cogs, etc.) this may not have held true.
Like you, I'm certainly no expert on this, but Hanson says they were longer range guns. Much of this was achieved by the Genoese and Venetian practice of BRONZE casting of guns, which gave them the ability to maintain a slightly larger powder charge. I do think the Galleass' guns were re-loadable. They simply could not have achieved a five-fold kill ratio if they weren't.