The two mentioned in the article met the FDA definition of remission. It doesn’t say, how many improved in general. This is important specifically since the sample size of the trial is small.
It's the important & useful of the two endpoints they measured. Anything will have a positive effect on depression if it comes with a lot of high-tech vodoo and people in lab coats. Only it won't last and it will the "yeah, I guess I'm feeling better"-level of improvement.
It's quite possible humanity has survived because most people never get depressed. You need to be quite clear about definitions to have a meaningful conversation on this.
Yes. And given the prevalence of continued prevalence of depression, there might be some evolutionary benefit for the genes that also cause depression.
No clue whether it's the depression itself, or whether that's just a side-effect of something else that's useful.