Can't reproduce. I downloaded your world and went to world spawn and dolphins, fish, and squid are spawning in the nearby ocean.
Sure, I just added a couple and updated the description.
Not fixed. Still occuring in 1.14.4
Yea, would seem weird if this was intentional.
Confirmed for 1.8.1
If you mean "no particles when breaking signs", confirmed.
Just redownloaded your world, and this time used vanillina and only enabled cheats using an NBT editor to rule out any changes due to Open to LAN or creative mode. I gave myself an elytra and firework rockets to fly to that same ocean and again ocean mobs spawned at that location.
It seems to me like the problem is not with the world but your environment. Do you have another computer you can test it on? If not, have you tried deleting the .jar file in the versions folder and redownloading 1.14.4, or even completely removing Minecraft and reinstalling/downloading (making sure to backup saves, etc...)