With regard to the rest of you, what is the reasoning for reading some prayers silently but the Anaphora aloud?
I'm not sure, to be honest. Some are clearly prayers for the priest alone, but I suspect the other inaudible prayers (which occur in the anaphora as well as outside it) may have been read inaudibly in order to save time while the congregation/deacons were singing something else. My only real basis for this suspicion, however, is lived experience: I've seen even some of the prayers usually read aloud taken silently in order to save time on weekday mornings before people have to go to work, for example.
Interestingly, the rubric, at least in Syriac, doesn't say that the prayer should be read inaudibly, but "bowed". If you are bowing while reciting a prayer, I suppose your voice becomes quieter or people find it tougher to hear, and this eventually became the inaudible prayer we have now (in Malayalam, "bowed" is normally translated "secretly" or "mystically", even if it is understood as "silently).
I ask because the Anaphora is generally the focal point of debates (in my experience with Roman Catholicism) over whether some prayers should be silent.
Yes, but in my limited experience, they have a totally different idea of why the Canon and other prayers are recited silently. They have theologised silence, identified the Mass exclusively with Calvary, etc. I am unaware of any such idea in our tradition regarding the importance of silence.