A Mute as a Witness

Tosafos in Gittin 5a mentions that a mute (an ilaim) cannot write down b'fanei nichtav u'b'fanei nichtam. The Ran (3a in dapei harif) argues that there is no problem, because b'fanei nichtav is no better than eidus isha.

The difficulty in the whole discussion comes from a Teshuvas Harivash (382 & 413) that says that there is no psul of mipi ksavam at all when it comes to kiyum shtaros. Yet, even the Rivash does not allow a mute to provide the evidence of b'fanei nichtav!

One way to answer this question is to say like the Ktzos HaChoshen in Siman 46:19 that a mute is actually pasul l'eidus due to mipi ksavam as a psul haguf. In other words, it's not merely that eidus must be spoken, but eidus must be provided by one who can speak. However, the ktzos himself leaves off with a question that, in general, psulei haguf are not a problem by the eidus of b'fanei nichtav. So, why should a mute be pasul?

Perhaps we can distinguish between the nature of the psul haguf. If the issue is one of neemanus (whether we trust the eid), we waive our suspicions when it comes to b'fanei nichtav. However, a mute is pasul because he is not "fit for stating" his eidus. This psul is not based in neemanus, but in his communication of eidus. This type of psul we have no reason to waive by b'fanei nichtav.