Application of laws is as messy, imprecise, and frequently hypocritical in this world as in real life 
I think, though, the parameters are pretty well established by the Connor storyline in Origins. The law says Connor has to go to the circle, and Arl Eamon will obey that. Isolde wants to conceal him. I bet that choice comes up in every noble family that produces a mage. The Trevelyans were like Eamon, and the Hawkes were like Isolde.
If it wasn't for Jowan and Loghain, it's entirely conceivable that Connor would be Arl now, assuming Teagan is advising Alistair.
The Kinloch Hold circle in Fereldan makes it pretty clear that mages can't really keep their lands.That's the most liberal circle we know about. The First Enchanter calls the shots more than the Templars. He routinely lets his senior mages travel on circle business, not just the conservative "aequitarians" like Wynne but even libertarians like Uldred. Wilhelm lives entirely apart from the Circle, yet is still considered a Circle Mage.
And yet even Wynne isn't allowed to keep her child.
I imagine nobility plays a part socially, of course. Like how in the middle ages, the bishops were chosen from the noble-born even though clerics entering the church had officially gave up noble titles.
Vivienne is also a good test of what a mage can do in terms of power outside of the Circle. She can be Duke Bastien's mistress, but she cannot marry. Marriage is about nobility and titles, she tells you, and she finds the idea of a mage marrying absurd. She's just about the most coddled circle mage around, and even she can't marry. Unless she's named Divine (a move that shocks everyone) the best she can hope for is Court Enchanter and various forms of unofficial power and influence.
One thing that hasn't been mentioned much here is the Court Enchanter title. It's the only alternative to circle mage/warden/apostate dynamic, and the only one that wields power outside the circle. The Stolen Throne established even before Origins came out that pretty much every prince of the realm has a mage in their court, and that the Chantry tolerates this (barely). I'm guessing that's what Wilhelm was (likely as a reward for helping the resistance).
I'd guess that most court enchanters in the various realms are nobility, though that's only speculation. It's the only outlet for a politically ambitious mage who isn't hiding their powers.