Andraste being a mage or not doesn't really matter that much for the Imperial Chantry.
It is not stated that she was in the Chant, and the Imperial Chantry has not altered the text, although it is said in WoT that they have a few more canticles or something. It is just how they interpret the text that is different.
While Hessarian was a convert, not sure he was the first. As it seemed her friends, that guy who took her ashes to Haven, and Shartan were disciples before him, he declared his conversion publically ten years later. Although, he was the first to set up an official religion around Andraste and declared it the religion of Tevinter long before Drakon was born.
On another note, this has really become like the Magisterium, where the magister all bicker amongst each other and only unite when the Qunari Support posters invade our thread or some anti-Tevinter southerner speaks up. It does seem there are some heret... I mean believers of the old ways or those concerned only for their own power, but that doesn't mean we all don't revel in the greatness of Tevinter.
Also, Hessarian was able to crush those who opposed him when he started the Transfiguration, which means the old magisters, altus, and old god priests were weak and were unfit to rule as chosen by the divine (being old gods or Maker), so Hessarian was the greater one and showed his worth as Archon. Yet, today we see many petty Archons who are so easily assassinated, even in their prime. Indeed, we should have an Archon with the strength and resolve of Hessarian again! He is the one who pulled the nation back up after the first Blight and Andraste's just war.