The revenge plot wasn't Oliver's idea
Major unmarked spoilers for volumes 5+
First, absent his mother's murder, Oliver was likely destined to be an ordinary mage who wouldn't ever measure up to his parents, and that was okay: he just wanted to be a comedian, and he was good at it. We also know that he was only seven or eight when she was killed.
Furthermore, Edgar Groves was crushed when Chloe was murdered, and he became a Crusading Widower who formulated Oliver's Training from Hell. And because of certain revelations in volumes 10+, we know that the Sherwood clan is just as rotten as the rest of the major mage houses.
So, it seems entirely possible that a young, impressionable, and grieving Oliver was tricked into pursuing his self-destructive revenge, not because the Sherwood clan had any loyalty to Chloe's dream, but because they self-interestedly couldn't let her murder stand unanswered, and are therefore using Oliver as their weapon against her killers.
- Volume 10 spoilers: Half-right. Eight-year-old Oliver was goaded into it by The Patriarch of the Sherwood family, but didn't take that much convincing after reliving her death through Chloe's Ghost Memory. The real twist is that Mr. Sherwood didn't actually care overmuch whether Oliver succeeded: he only wanted to see how far he could take the soul merge without dying and to breed Oliver with Shannon to keep the magic in the family.
