well, what title did you have? The text of "Across the Summer Sea" says that you cannot initiate challenges or have them initiated against you. If the attack is being redirected, I am of the understanding that the Summer Sea player is an eligible target for the redirect because the challenge was not initiated against that player. I may be in the wrong on this particular issue, but have had the Kingsguard title with this plot and successfully redirected an unopposed military to myself for this very reason.
You are incorrect. Crown Regent's redirect takes place as part of initiating the challenge. The challenge is considered initiated against the new defender. Lord Commander's redirect happens well after the initiation is over and done with. The timing of their redirects are vastly different, so what applies to one will rarely apply directly to the other.
To more directly answer your question, if you are not, in fact a valid target, due to your plot, then the crown would do nothing. You cannot "switch between titles" in this way. You have both, so P1 is both supported by master of coin and has the ability to redirect. Once you have the title(s), you can't pick and chose which ones are in effect at that precise moment…. so A)
It's like having a card with more than one House affiliation. You can never say Brienne is "just" a Stark character, completely ignoring the Baratheon affiliation that is also printed on the card. She is always
as much a Baratheon character as she is a Stark character.
Similarly, the guy with two titles was always as much Master of Law as he was Crown Regent - having all benefits and drawbacks of both cards at the same time.