So, epoxied part back in place and will make the correct cut once dried. As you can see from the picture, cut was not even close!
Plan B on bulkhead 1 went much better; cuts made with handsaw and final fit with chisel. Yet another oops shows itself here, but angles are good and epoxy covers all.
What's worse, these mistakes were made while 2 brains were present...My dad had the afternoon to assist and offer encouragement.