Get legal advice.
If it goes to court because of the overnights, if you really suspect drug use, then you can request drug testing. But, he can also request the same as you.
Contact and overnights does not have to be a school/nursery night. It can also be at the weekend. This will add to the quality time.
I would not minimise contact, because surely this will annoy the ex, who will then seek legal advice etc. Seems to be the norm that courts give alternate weekends, christmas, birthdays, holidays etc.
I wouldn't stop communicating with the ex either. Why would that even be considered? With the exceptions of abuse, parents should be communicating with each other like adults. Afterall, it's in the best interests of the child to act like adults, and put their own feelings aside.
Usually, the only time overnight contact is prohibited is when the absent parent has abused a child. This is on the recommendation of SS.
Parents have parties around their children and not just the childs birthday parties. Drug use is no excuse tbh, and gone are the days that users are no longer allowed to care for their children, unless children are at risk. People have sex in front of their babies, just read countless threads on here where babies are in the bedroom in a cot, parents have sex and get pregnant.