I developed a VM project just like this one. Asked Fable to stress test it and try to break out of containment, and to my surprise it didn't manage to. Fable didn't get downgraded to Opus either, for some reason.
Would have thrown Mythos at it if I had access to it.
You should conduct the same test with knowingly faulty containment, otherwise the theory that the model is hobbled should probably outrank that it couldn't escape.
Isn't Fable just Mythos + guardrails? Sounds like you did throw Mythos at it.