I have never seen a ship respawn after one has been sunk in a convoy. I have seen the convoy 'close ranks' to fill the gaps. Now, there have been instances where a ships icon reappears on the nav map but this usually occures after you leave visual range and receive a radio contact report.(you will see a sunk icon with a ship icon on top) This is a stock bug.
As for the sinking of ships after 3-4 deck gun rounds, this surely depends on what they are carrying and where they are hit. As far as I can tell, things like this a rare and almost random. I have actually sunk a small merchant with one 'lucky' round but that was in beta testing and I'm pretty sure Von put that right.
