Its the fact that they have different salt densities. So they just physical can't mix. The difference in densities I'd kind of like water and oil. The oil is denser to the water floats on top. Its the same here, one ocean is denser than the other.
No they would still mix. The salt would just evenly distribute its self across the ocean. They only reason there are places with different salt densities is due the temperature. In a hotter climate more water is evaporated while the salt is left behind. This means that there is a higher density of salt.
Oil and water do not mix because oil is comprised of non-polar molecules while water is polar H2O molecules. Non-polar solutes cannot be dissolved in polar solvents. The difference in densities (water is heavier than oil) means the oil forms a layer above the water.
About the oceans mixing, I have no idea
About the oceans mixing, I have no idea