I'm not too sure of the mechanics of it all to be honest.
I believe aggro'ed Drowneds can also summon other Drowneds. However with no AI, I suspect this can't happen.

 

It’s true! It’s why Plains superflat and other certain Minecraft modes can’t get tridents. (Some now can because of trial chambers). However, it seems like the mob stacks (at least from mob spawners) don’t realize this and will treat a mob stack of zombies that drowned as naturally spawned drowned (after killing the first drowned, maybe).
Hopefully that makes sense.

EDIT: Of course, that won’t be a problem on here since we get Drowneds with tridents spawning naturally anyways.

 

me (true)