The world's freshwater need is about 950 cubic miles a year. (https://www.wolframalpha.com/input?i=worldwide+water+use+in+...)
You can just put the leftovers back without worrying much about it.
Yes, just not all in one place/time. Separating seawater into pure water in one place and pure brine in another, you don’t want to let that brine out all at once in one spot, it’ll kill a lot of ocean life. Most desal plants that are attempting to do this right, will pump the brine into pipes that diffuse it over a wide area to avoid oversalinating. And it still kills a lot of ocean life.
Or we could have a premix station where we pump in sea water and mix it with brine at a certain ratio and then return that to the sea.
And with your second process, you still end up with a higher concentration of salt / brine around where it's returned to the sea, still killing animals.
I'm thinking of pumping the brine onto large evaporation lakes and harvest the salt or whatever - which also already happens to produce sea salt.
And if there's too much salt for the market, just stockpile it. Like underground salt mines.
The big problem with making salt from sea water is evaporating the water. It either takes a lot of energy, or a lot of time and area.
It takes a lot of time for the salt to just diffuse out over the ocean.