I dont know of anyone who questions the accuracy of the Vulgate or Peshitta translations. You even praised the Vulgate at one point I think. And it says “Ave, Gratia Plena,” which is a perfectly valid way of expressing “Thou art highly favored” which is how other translations have it.
The Douai Rheims, Tyndale, Coverdale, Lamsa and other Peshitta translations are also way more reliable than the third edition NIV, which substitutes male pronouns for gender neutral pronouns and has other liberal biases. The second generation NIV also had issues, but the third generation is a problem.
And if Mark is right that the current Ave Maria is of Tridentine origin, but there is a possibility you aren’t, because the Eastern Orthodox versions, including the Old Believer version which predates Trent by many years, and has the intercessory prayer, as does the Syriac version.
However, I think you are probably right, right in that the intercessory prayer was added to the Roman Catholic version at Trent, and given your high level of accuracy on historical facts, I think the idea that at Trent, the Counter Reformers chose to add the intercession, for they were aware that the Syriac Orthodox and the Russian Orthodox had an intercessory prayer, due to Jesuit surveillance, and also the large Orthodox population in Venice, and the fact that the old pre Vatican II Maronite liturgy was almost identical to the Syriac Orthodox liturgy, and its possible the Syriac version of the Hail Mary fot in either directly through the Maronites, or was discovered via the massive amount of Syriac scholarship done by the Assemani family over the centuries. Just as the name Sebastian Brock is synonymous with Syriac studies today, Assemani was synonymous with Syriac studies in prior centuries. As for the EO version of the Hail Mary, that almost certainly got in as a result of the Union of Brest, which led to the Orthodox of the Polish Lithuanian Commonwealth, including Belarus and Western Ukraine, and all of the Carpatho-Rusyn or Ruthenian Orthodox, becoming Byzantine Catholics.
And I told you where to find it.