I would do #3. However I would cut out the existing backing plates and replace them. I haven't installed the one piece gudgeon on a boat yet. Does it use the existing holes or are they all in new locations? If it's all new locations then you can drill and tap. If you can't get the bolts out now you won't have any better luck from behind through a port. To cut them out a dremel tool works but I prefer using one of the sonic tools like the Rockwell sonicrafter. Works great and easy to control with less dust. You can buy 1/2" plate stock by the foot from McMaster Carr. I've also simply knocked them out by using a punch from the outside and just drive the plate out. Generally that plate is only covered buy a single layer of light cloth. To prevent this from happening again coat the screws with Tefgel.