We simply are not going to agree as to your fundamental premise that marijuana must be legal because alcohol is legal. That makes no sense to me, and especially not in view of the substantial and increasingly well-documented harms of marijuana. And, by the way, the study that you cite does not contend that alcohol use causes schizophrenia.
More of your straw men - that is not my fundamental premise, but as I said is a conclusion based on the following facts: "their bans are comparably harmful, as are the substances themselves: alcohol is more addictive than marijuana, more violence-increasing, and the only one of the two that can lead to fatal overdose."
That makes no sense to me, and especially not in view of the substantial and increasingly well-documented harms of marijuana.
"Harmful alcohol and other drug use, particularly cannabis and amphetamine use, may trigger psychosis in people who are vulnerable to developing schizophrenia. While substance use does not cause schizophrenia, it is strongly related to relapse." - https://www.health.gov.au/internet/publications/publishing.nsf/Content/mental-pubs-w-whatschiz-toc~mental-pubs-w-whatschiz-cau