Agree with you that Mayo is special, and that if I were going to be on one of our players being an impact, middle of the order bat, it's him (though if Cowser and Westburg continue to hit anything like this we'll have a few of them! And I'm a big believer in Kjerstad's bat too).
I don't think it's really that complicated, though, no? Santander will leave next season and Kjerstad takes that place. Trade Mountcastle and that's Mayo's spot. Basallo is still a ways off and things will probably work themselves out with injury or what not by then. Norby, Stowers, and Ortiz were always going to be the three that it was going to really be a struggle to find a spot for, and the Ortiz trade solved the Ortiz issue, and now I think you're in a situation where, barring injury, Stowers probably ought to be trade bait and Norby could become a really valuable 12th hitter off the bench next season.