Regardless of the safeguards in place, Efaen could have still done what she did. All that required was the morphing technology and a bit of xenophobia. It's more the aftermath that didn't necessarily fit, since she was supposed to be sending regular reports to Ossanlin and, obviously, could not do so once her memories were wiped. So he should have found out about the memory wipe sooner than he did (or at least that something was up) simply through the lack of reports.
But if we had made that pan out I wouldn't have been able to rp her at all...Ossanlin would have collected her immediately, and she would have probably been re-indoctrinated back into Andalite society and eventually ended up in exactly the same place that she was. I wanted her character to develop, rather than to go back to the same place in a different way, and that required exposure to the rebels after the memory wipe.
Of course that's all moot now. Ossanlin did collect her eventually, so short of this rp here I have no Efaen, and Marie no longer has access to the Yeerk character that infested her. She's...idk...on the Homeworld somewhere? Did we ever decide officially? For the purposes of this rp I presumed she was sent to the Homeworld and then eventually called upon to go to Corvus due to her knowledge of the rebellion, but I'm not sure if that's official or just for this rp.