I see what is painfully obvious. You choose to ignore the obvious to fit your best case.
The Mariners want to reduce payroll because they have a budget of $15-16M to find an upgrade at 2B, 3B, and BP. They aren't taking on more payroll to fill 1B while opening up a spot they have to fill in the rotation. Even the Bellinger rumors include Hoerner in the deal. The only place people are talking about Bellinger for Castillo straight up is here.
I challenge you to go read that MLBTR article that name drops Bellinger as a Mariners target and then get back to me with how realistic it is. The whole article is based on a Seattle times reporter writing about how the team is looking for a platoon option to pair with Raley, how Bellinger's contract is a big problem for what the Mariners are trying to accomplish this offseason, how they "covet" Nico Hoerner, and how trading from the rotation is "plan Z" according to Dipoto.
Don't know about you, but Bellinger really strikes me as the guy to get Dipoto to act on plan Z, instead of the player they "covet."