Because of Minrathous, They suceeded in couqering the rest of Tevinter just not the capital, its because it is extremely well fortified and has storage for enough food to last for years, It also has golems. The Orlesians had them besieged and were prepared to keep going and continue the war against Tevinter but then the fourth blight broke out.
But even to this day the Tevinter Magisters don't fully understand the Templars abilities, they don't have proper Templars in the north. Theres a war table mission called The Tevinter Resistance where one of the options is to send a squad of southern Templars to protect the Inquisitions Magister ally from her rivals in the Magisterium. When they attack her they are taken completely unawares by the Templars abilities, and the Templars proceed to kick their ass.
Oh awesome!
What were the requirements for that quest?
As for the topic question, i think it would be really really close.
The templar would have to close any gap between the two really fast as if the mage can keep the distance and pepper the templar with spells, i can see templar losing out.
However, if the templar rushes in, imbues their weapon with disrupting energy...gameover for the mage. And like what happens in Asunder, a templar doesn't even need a weapon to disrupt a mages magic.