> Since this led to a lot of local interpretation and variance (many -- perhaps most? -- priests were illiterate at the time) by 1000, I believe, the popes started insisting that the priests use Latin and turn their back on the audience.
It a congregation, not an audience, and in Roman practice before ~8th-9thC it was the audience whose back was to the priest. In both Eastern and Roman practice, services had both the priest and the worshippers facing East; in the East the priest was at the East end, facing the altar at far Eastern extremity, with the congregation facing the priest and altar. In Rome, the priest was at the far West, facing the altar and, beyond them, the congregation.) Later, the Eastern approach was adopted as the norm in Rome and the West (and eventually the invariable East-West orientation abandoned), but in neither were the priest and congregation facing each other as in the modern Roman mass
The universal adoption of Latin (and the Roman rite) in the West was, IIRC, driven more by Charlemagne and subsequent Emporers than the Popes.
