This is what fear shrouded in bluster looks like:
Why should Trump sit for an interview with a man conducting a witch hunt? Has he been accused of any crime? Is there any probably cause? He has the same right every American has not to talk to law enforcement. Considering the breathtaking plot carried out by the DOJ and FBI against him and his campaign and their efforts to invalidate his election, no reasonable person would expect him to cooperate.
Rather than get bogged down in even more legal gymnastics over grand juries, etc, he should send someone over to Jeff Sessions' house to see if they can rouse him from his coma. If not, put him out to pasture and appoint a real AG, who would then terminate this ridiculous spectacle.