I doubt very much that the people Meeran or Athenril had Hawke fight were trained templars. In Act 1, someone looking to score brownie points with the viscount (or more likely, Meredith) would just have to advise the templars to come in force. I don't care how awesome Hawke is, superior numbers (who also happen to be well-trained and -disciplined, not to mention able to specifically negate a mage's power) will overwhelm.
Also, would the templars coming to arrest Hawke really say "We were tipped off by the Reinhardts that you're an apostate!"? That's... stupid. That would be them throwing their informant under the bus, giving absolutely zero incentive for other people to snitch for them in the future. Sure, it could be a trap and they'd give the informant a protective detail in order to catch more malcontents, but... Come on. People are currently arguing that mage Hawke is believable because the templars are incompetent, so we can't go turning them into the super prepared planners of genius.
Sounds like a lot of trouble to go to to bring down the authorities’ attention on your own business.
Let’s break it down: first someone needs to notice Hawke’s an apostate. It’s unlikely to be that noticeable, as we know Hawke’s family lived for years in Lothering without been caught and sent to a Circle. Hawke’s not a freshly-escaped Circle mage with no idea how things work outside the Circle, they’re part of the outside. They’re good with people, which we know from the merchants greeting them by name and promising discounts when we start Act 1. So Talkative Man or Drunk Patron are unlikely to be the ones to notice Hawke being a mage.
Someone who sees Hawke more often, then. Perhaps one of Gamlen’s neighbours? So advising the Templars to come in force might mean an all-out fight right where you live. Do Lowtown citizens trust Templars to avoid casualties? Let’s assume everything goes right. Let’s assume the neighbor knows to tell the Templars they’re going to need more than two people to bring Hawke in, and the Templars listen, and Hawke doesn’t get away, and you’re not a casualty in the resulting battle. That’s a lot of assumptions, but okay. Then you’re left with Carver Hawke living next door. How likely is it Hawke’s little brother is going to let the issue drop? That Carver’s an angry sort, who might try and track down the one who turned their sibling in. Would you really bet your life or that of your family on a nineteen-year-old with a chip on his shoulder not carrying out their own justice? (Ooooh, has Hawke taken a loan from Donnal? Cause that’d be precious incentive not to turn them in. You wouldn’t want Donnal sending legbreakers after you.)
And even if no-one finds out, the Templars know. Templars who go to the Blooming Rose, who gossip – and who know where you live. Informants can be blackmailed.
Incidentally, the game makes very little mention of informants for the Templars. IIRC, for the whole of Act 1 and 2 the only person we know informed the Templars someone was a mage was Arianni, and that was about her son. Turning people in for being apostates just doesn’t seem a big part of Kirkwall culture. Again: Anders is a mage much more openly than Hawke and no-one turned him in yet.
Ultimately it all comes down to Hawke living a year, a year and a half without being arrested by the Templars. They did more in Lothering.