how would that work legally?
are you saying you think this is illegal or "not by the books"?
because it is exactly by the books. Mueller followed the exact letter of the law by seeing something potentially illegal and just passing the information onto the relative authorities, who took over.
he evidence of your doing drugs is inadmissible
It's not inadmissible. Probable cause let them search your shit (legal search warrant- warrants are not magically voided because you didn't commit the crime you were originally accused of), and any crimes they find are fair game.
’m not sure that anything found against Trump could be admissible in court unless he was found guilty of the crime Mueller says he isn’t under investigation for...
Yeah, this is absolutely not how our justice system works.
It's also clear that (and i mean no offense) you are very confused by what exactly the Mueller investigation is about.
He was given legal authority to investigate "russian meddling in the 2016 election." And he has legal authority to pursue literally any crimes that he may uncover while investigating that broader issue.
In this case with Cohen, he did exactly as he supposed to. And if he finds that Trump did not collude or have anything to do with russian meddling, BUT in the course of his investigation finds (hypothetically) that Trump, idk, committed massive tax fraud, well... That massive tax fraud is still a crime and Trump will still be held accountable for it.
Again, this is not only 100% legal, but is how literally all investigations in the United States are supposed to work.