User blog comment:FairyRaven/Fairy Tail Chapter 456: Orders, Review and Commentary/@comment-24546844-20151011013708/@comment-25111584-20151011020910

Not being able to take over an Etherious is more of a testament to Seilah's strength rather than an intrinsic property of the Etherious, unlike Mira's immunity to Macro. I thought that was quite clear. ._./

And, while there's no problem with finding Seilah's face making a cutesy to be strange, it does fit the situation as that is just Mira using Seilah's body, not Seilah herself, who is inactive inside Mira.