Can the President Be Arrested?

If President Trump incited unlawful insurrection at the Capitol (I think he did, partly because his being President amplifies his speech), here are questions:

A President cannot be arrested while in office. But can a former President be arrested for an act done while President?

Can a former President be sued for an act done while President?

If the answer is affirmative for either, can the former President be found not guilty or not liable because the alleged incitement, especially if vague as to what the incited might subsequently choose to do, did not exceed what is allowed by free speech protection or by the President’s discretion in fulfilling the rights and duties of the Presidency?