This isn't my argument I am just gonna back him up a little on the two things you seem inconvinced of for sure.

There is more I'm worried about than fish dependency on lithium as current studies (though there are few) show that most sea life, biologically anyway, have a little too much in them currently, mostly around the brain, but too little in the muscles. https://pubmed.ncbi.nlm.nih.gov/32882547/

The real issue (that we currently know of for sure) with solving lithium from the ocean is noise pollution, which sea life is extremely, extremely sensitive to. I'm not going to provide a source for that, watch fish die at your own leisure. I don't vibe with it. (You've honestly probably already seen a few videos on here anyway)

Now mining and quality of the metal. That one you are the big wrong on. Melting it down doesn't make it "pure", and without what would be an expensive and dirty process to chemically drop the lithium out to claim it on its own (like you mentioned with the layer added on to desalination) you'd get a loss in quality for each step you shortcut for cost savings and efficiency. There is absolutely a standard of quality we use for every metal for every job. We will not use steel borne from pig iron to construct a skyscraper for instance (atleast you really really shouldnt), and lithium is a spicy metal, a lot more can go wrong with that if you don't do it the right degree within x% of contaminates.

You both are right. Lithium is by no means exceptionally rare, but it's going to take a lot of money, care, consideration, and time to do correctly. Most people riding the electric trend hard refuse for time to be allowed, most people against the ev trend would refuse to give it money, care, and consideration.

Again, no sides taken, I just like to hit hot steel with a hammer and make things and have dropped lithium out of a few different solutions via electrolysis.