Build subsurface wells and responsible brine dispersion infrastructure then come back and tell me we can desalinate as much as we need to.
If you put the brine back into the sea, and later put the waste water back into the same sea, doesn't it balance out? Also, the sea is pretty big.
Perhaps, but locally, the higher brine concentration will cause issues.
Yes, hence my use of the word "dispersion". Over a wide enough area, the brine shouldn't have a noticeable impact on sea life. But concentrated release can be really damaging.
Nuclear reactors regularly shut down because the water from the nearby river is already too hot.