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One thing I am certain about is that trump wants a show down with Mueller.
He wants to break Mueller, the FBI, and justice department. Maybe he will attempt to do that during a face to face interview, or do it in some other way. He wants to be able to show the world that he can not be defeated, and all those to challenge him, walk away as losers. trump loves being the dark horse that always wins, even if winning means using methods\tactics others refuse to use. If the other team is bigger, stronger, and more skilled, then put poison in their food; do whatever it takes.
Deep down, he knows the worst that will happen to him is he goes back to his previous life. He won't go to jail, and he has power of pardoning anybody he wants. If he were to resign (IMHO, 90% chance he will), don't be surprised if all the big names associated with him are pardoned at the same time, 20+ people all walking way with their criminal record cleared.