I screwed up by adding a rocky shoreline outside island boundary. How do I get the boundary moved to encompass this extended shoreline?

As mentioned by the title, I'm cleaning up the shoreline on Lake Superior. Here is my working example of the problem.

I added the rocky shoreline as detailed as I could and made the new rocks as a relationship with both the island and Lake Superior. Everything looks fine on the map but I was told by an admin that this was the incorrect way to do it as an object can't be inside two multipolgones at the same time. The better method is to extend the line (island boundary) around my new rocks. The problem is the time to adjust each point manually. Is there a possible automated way to do it?