It depends on which State you live in, what the powers of sheriffs are.
In PA, sheriffs are not law enforcement in practice, they serve as court bailiffs, run county jails, serve subpoenas and civil suit papers, but cannot arrest people unless they are in an immediate situation where no police are present and a person needs to be detained for public and personal safety. Nor can they issue traffic citations - they can't pull someone over for a violation.
In liberal logic, police enforcing the law and arresting people are violent acts which incite further violence from peaceful protesters.
No, he's never going to send in the NG unilaterally.
He's laid out the case that "stupid rich people" (from Ingraham interview) are funding Antifa and BLM, and it's within the power of those stupid rich people to call them back; also within the power of the mayors and governors to stop the violence, and it's in their court. If they fail to put an end to it, it's all on them. The President is under no legal, constitutional obligation to police the streets; the 9th and 10th Amendments make that clear - it is up to the States and municipalities, and only upon their direct request will the President step in to lend them a hand in quelling the violence.