Nightmare, there's a difference between access and control. Consider the folliwng parallel: magic is like a fire in a fireplace. An adult can both access and control the fire in the fireplace; they have knowledge and experience, know what to do and not do in order to prevent it from going wild. An experienced mage, even one who's half-burned-out due to implants, has both the knowledge and the experience to generally control their magic. A child, a toddler, on the other hand, has access to the fire, but has no real control over it - they don't have the coordination, the knowledge, or the experience to make it do what they want it to do. This is like an ELINT taking over a person with active magic - they have the access, but not the control. Any knowledge they may have is completely theoretical, or acquired memories of 'how to' at best. (Ask someone who hasn't walked in six months if they remember running - they of course do. Make them run, however, and disaster results.)
The 'Sybil' personae accessed the magical abilities just fine. They lacked the control, though, and ... kaboom.