That would definitely do it!! Reducing drains like this is a big no-no, for exactly this reason!
The shop vac might get it out… if that fails, my though was hooking with a metal wire inside the shell opening to pull it up and out!
Either way, I’m glad you’ve discovered the issue!
That photo is priceless, by the way! It’s the worst-case scenario I’ve always thought about, but never seen!
It should be noted that many marine snails will happily climb above the water line, allowing them to climb over weirs with enough overhead clearance!
— I’d say weld in a replacement weir tooth, then put the screen cover over the top of the weir!