Choices

[Mira]

You forget:  Michael has some extra guidance! 
Without Harry's assistance, SI & Murphy will have needed some extra help... help that Michael is well-situated to provide!  Obviously, that's no guarantee Jim will have written it that way; but it'd be  a nice exemplar of God's Providence, with Michael's multiversal presence situated to support Murphy (or in a different Alt!Universe, Morty?) when that alt!Harry does not.

Unless Murphy is closed minded about the supernatural, there is no guarantee that she'd be receptive to a so called "Holy Knight."

I don't see any way for Mirror!Murphy to have buried her head in the sand and deny the supernatural; and Harry will *very* much be on Mirror!Murphy's radar -- as a Bad Guy(tm)!

Actually you make my point very well.. Murphy sees things in black and white, if all she sees is an "evil Harry" how do you think she will see the supernatural in general?  Also remember how quickly she was to jump to conclusions in the first couple of books because she did not understand Harry's world at all.

[g33k]

... there is no guarantee that she'd be receptive to a so called "Holy Knight."

Michael doesn't often introduce himself as a "Holy Knight," though:  he just steps up to help.
And Murphy actually has amazingly-good "people-sense."
It'd make good sense in both Doylist & Watsonian terms.

If Michael is guided to aid her in time of need -- and I allege that (in an Evil!Harry universe) this is likely to happen -- Murphy will take the help, and (cautiously) begin trusting Michael, as he becomes a good source of supernatural intel & an ally in protecting Chicago.