Question: Why would the town turn on Laurie and accuse her of provoking Michael's attacks when she was a victim and spent almost the entire time in the hospital?
Answer: Because the town, Haddonfield, needed someone, anyone, to blame, and Laurie, being that she survived both the '78 killings and then the 2018 massacre, was likely the only viable choice.
Question: Michael dragged Corey into the sewers with the intent of killing him. Why did Michael let him go?
Answer: The implication seems to be that there is some sort of subliminal connection between the two that they both pick up on. I personally took it as Michael somehow subconsciously feeling the "evil" brewing within Corey... almost as though he sees his own evil in Corey. Thus, he lets him go and subsequently "teams up" with him later on. They're a twisted form of kindred spirits.
Answer: Michael's primary target is Laurie but, since he's also going around killing random people, most likely everybody believes that Michael is killing anybody who gets in his way to get to Laurie.