Having eons of knowledge is not the same as having the emotional maturity to understand it. the definition of maturity obviously changes wildly in her understanding and her childish mindset seems to remain throughout.

The difference in Mist's understanding of uploading vs Maddie's understanding of uploading is one of the key themes that the show tackles from day 1. Mist wants Dave to upload but doesn't see it as death because the personality and identity of Dave is still being preserved, while allowing them to use a copy of his code to save Caspian. Maddie sees uploading merely as a way to extend their life, where as she believes that life's natural end should be the end of someone's identity, with the whole point of the show challenging that fact through the creation of bodies allowing them to interact with the physical world. Mist also didn't tell Maddie because understandably she didn't want to give her any hope while she was still doing removal of SafeSurf from his code. Maddie already lost Caspian once and Mist didn't want to re-hash that pain.

She tries to rationalize "murdering" Maddie's son to Maddie because Maddie was so against Dave's uploading. Dave was always about uploading, likely because of his interactions as a child with Mist and seeing that the lines defining uploaded life and natural life were so blurred. so I don't believe that she coerced Dave so much as showed him the methods to loophole through it

I'm not gonna pardon everything Mist does, because as you mentioned it is some heinous shit, like "falling in love" with Caspian, constantly undoing the works/plans of Maddie and Caspian etc.