Good points.

For the salt amount look at the dead sea and you will see that it can get high enough to kill the fish.

"but the incredible amount of water flowing (enough to create the grand canyon, for instance) would probably make the oceans extremely salty extremely quickly, hence making adaptation quite unlikely."

It would be all a matter of currents I suppose, but your point is taken. If it happened quickly enough it would kill them. If the water rose slowly enough this probably wouldn't be a problem. However your point is worthy of further examination, I will look into it.

-Aaron