Cat...as you've said, you're basically looking to replace the port-side stringer. For ease....how about a combo of the ideas you've mentioned? That is, on the replacement stringer, forget the mold ($$$) and go with coosa glassed-in to form the new stringer, foam-filled, with a "permaloy" synthetic top.
Basically the same as what you have, but stronger, with the ability to place screws in the permaloy. Will be less money than taking a mold off the other stringer. And since the deadrise angles are reversed, a potential can of worms. Plus...a bear to get the deck height correct side to side.
Probably easier to just run strings, and build to the strings.
Best wishes for the upcoming school year