Using your deck gun against DD is the quickest way to end your career. Good thing is, you'll be remembered in British pub's for some time
From my experience (stock version, patched), destroyers don't mind endangering merchants - they urge to kill u-boat. I was ca. 50m from a merchant, when DD came out of nowhere shooting like a madman. I crash-dived, but he was able to put at least 3 rounds in a merchant, unfortunetly not enough to sink it. My boat was unharmed. Judging by this event I'd say convoy is not going to protect you. Plus, in a middle of a fight you can get rammed by enemy ship.