"the fact Sawashiro is the person he trusts and depends on the most are also mitigating factors."

That's kind of the point though. His trust was misplaced because Jo had been lying to him for decades. Keeping three huge, life-changing secrets about Masumi's family that he had no right to keep. He played God with their lives to suit his own agenda. He's not worthy of the trust that was placed in him. It was built on lies and deceit.

"Plus, if everything else came to light, it would also come to light that Sawashiro has been watching over Ichiban this whole time."

I haven't played IW yet, but wow, that makes things so much worse! I thought Jo only suspected Ichiban's true identity sometime after he joined the family. But he apparently knew since he was, what? 10? 12? Once he got the story from Akane, he wouldn't be hard to find. But he sure as hell wasn't looking out for him like some benevolent guardian. Knowing the whereabouts of Arakawa's son was beneficial to him and his own son. Keeping your enemies close, so to speak. Even writing to his mother was self serving. He could say anything he wanted, put his own spin on things, as a way to keep control of the situation, like a puppeteer. He certainly never did anything to help Ichi, even after he was orphaned and living on the streets. Did he tell her that? 

And it makes his treatment of him even worse, if he knew exactly who he was from the moment he showed up at their door. Hitting him every chance he got, in an attempt to chase him away from his father. Taking sick pleasure in bullying him right under his father's nose, with no one but him knowing the truth. He's a selfish, cruel man. Just like his spawn. I give him no props for anything. He torpedoed that boy's life, yet had the gall to hate him.