Isn’t impeachment required before a sitting President may face criminal charges?
Yes. It’s why the grand jury investigating Watergate related issues named Nixon as an unindicted co-conspirator instead of indicting him.
But can you impeach a person who holds an office fraudulently? If he was never qualified to hold the office, then he is not officially legitimately the President, having been elected under forged documents and false premises. Therefore, all appointments, all bills signed by his hand, treaties negotiated, etc., would, and should, be null and void.