In my humble opinion, it's not far-fetched that Osha joined Qimir after he killed all those people, including Jecki and Yord. I loved Jecki and Yord. But looking at things from Osha's perspective, their deaths didn't matter as much the second she learned about the truth. The Jedi were responsible for her mothers' deaths, the death of her coven, the separation between her and her sister. They. ruined. Osha's. life. As Mae pointed out, she lives a life without purpose because of the Jedi. No matter how "noble" the Jedi's actions were, the consequences were devastating. Jecki and Yord were a part of this institution that destroyed her life. They weren't involved in the tragedy, but they are Jedi.
Osha only knew Jecki for what? A few days? A few weeks? She was a great Jedi (I reiterate, I loved her), but at the end of the day, Osha barely knew Jecki. You could argue that she knew Yord longer, but she hadn't seen Yord for like 6 years. When they met again, their relationship was very different (mainly because of Yord). Osha doesn't give a fuck at this point.
As for Sol, it doesn't matter that Sol had the intention to confess his mistakes to the High Council. He chose to stay silent. And he did so for 16 years. And he continued to lie to Osha as everything was unraveling. It wasn't until Qimir questioned his integrity that Sol felt the need to come clean.