I've always seen it as that Ella wants us to win and get strong, but if we burn out quickly and die, then we weren't worth the time investment. So as the MC evolves and gets stronger, she allows herself to become more relaxed and happy around the MC, as she feels more comfortable trusting him to become the kind of person she needs him to be. Which I feel works well with both aspects of her personality. She is more than willing to drop the MC like a bad habit if she feels he isn't living up to her standards, but when he does, then she can be more friendly and happy for his success.
Jake needed the help because mentality is such a big barrier to strength; if his mind wasn't in the right place, he wasn't going to gain the strength he otherwise could. Here's a big sticking point, in that after Syla died, the MC ran away to join Ella's group for a month or two. During that time, Ella could very easily have taken stock of his mental state after the trauma of his mom dying, even if she erased his memory of it, and assessed that the MC has the mental fortitude to excel as a superhuman. Jake, on the other hand, very desperately needed all of the hand holding he could get, and even then he still categorically fucked up. From Ella's perspective, the MC was going to overcome whatever challenges she sent his way, and "it is better to ask for forgiveness than permission".