It depends how they evaluate the pitching left on the free agent market. They have plenty of data and personal experience with Bauer from Cleveland, so if they passed on him I wouldn't question their decision.
It's unlikely but whose to say we can't do both Realmuto + Tanaka? Our payroll is just roughly over 105 million with our current signings.
Signing a guy at a position of strength and then dealing from depth gives us flexibility to target a larger pool of players. I would compare it to when the Leafs signed a goalie in Cujo despite already having Potvin. They were then able to flip Potvin for a top flight young defenseman in Bryan Berard (who tragically had his career cut short).
If Jansen + for Castillo were possible, then we'd be left without a starting catcher unless we signed or traded for a guy. Moreover, according to my calculations (using Fangraphs models) Castillo is worth between 75 to 78 million in expected surplus value. The closest players we have to that much value are Jansen (around 65), Gurriel (45), Pearson (44), and Martin (39), so Jansen would make a lot of sense to move in a deal for a top level pitcher if we could free up a spot.
Then again a rotation of Bauer, Ryu, Castillo, Pearson, Ray would be mighty sexy. We'd just have to settle for handsy McGuire as our starter until Kirk or Moreno are ready.