She can just say shes in slumber, waiting for the right time.
Or got amnesia and looking for her home/kin.
Technically she can't, since she can't talk without killing everyone in hearing distance.
Also, while she could write out replies, Japanese is apparently the Ancient Language, so there were probably Reincarnators from Japan way back in the past, but it's likely that only academics and specialists would be able to read it.
But addressing the idea of amnesia/loss of memories, sure, I think that could be a good excuse.
The (wrong) thought process that Noel and others could have would be, "If she gave up her voice for immense magical power, then what kind of power could she have gotten in return for giving up all of her memories".
It's a bit of a cop out, but she could explain away everything as "traded away for UNLIMITED POWER"; her voice, her memories, her ability to eat solid food, and even the fact that she can't touch anything living without killing (absorbing) them.