It’s complete speculation. There is no “proof” for it.
Yes, and they make perfect sense the way the church ordered them. There’s a reason Mark is not first.
Our religion is not a philosophy - it’s based on actual historical events - the incarnation, ministry, death, and resurrection of Jesus of Nazareth. Modernism has done everything it can think of to dismantle our faith in the veracity of those events - and Marcan Priority is one avenue of that attack. Liberal theology says, the historicity of the gospel is irrelevant - what’s important are the ideas, and specifically how they relate to social action.
Also, “Q” is necessary for Marcan Priority. You cannot have one without the other - which is the biggest indication that it’s a flawed theory.