Couple things...first off, great forum topic! I can’t believe you started punching the fish! I laughed out loud after reading that. "That poor fish" :)
I have heard of lots of issues with bigger fish eating a diver’s catch. My philosophy is...if the fish is bigger than me OR has more teeth than I do, it can have my catch.
And regarding the anchor line...I would have gotten out my dive knife and cut it. I hate when other boats don’t respect the diver down flag.
Congrats on your first open water dive, sounds like you had fun!