#1 is huge and compounded with the caveat in your asterisk could be even bigger. We can’t predict how behaviors and supplies would adapt. There would still be an illegal drug trade across the border. Untaxed and unregulated is cheaper. That some problems may be lessened isn’t terribly compelling.
I would challenge #4. I see no reason not to treat it like a public health concern now, when mere use could be used to compel rehab. That would be much smarter than sending someone to prison in many cases. Treating something as a public health concern doesn’t mean there is a great rate of voluntary treatment.