Every society that has had free mages has always placed them in the position of rulers. Tevinter, Dalish, Rivain, Chasind. They may not all share Tevinter's ideal but they all granted freedom to their mages and they all ended dominated by them.
This ocurrs because mágic is an advantage against which is extremely hard to compete. This advantage will be used to earn influence and wealth for the mage social class and it will, inevitably, lead to mages controlling society.
To believe otherwise is to be naive.
Out of that four examples, only Tevinter can be objectively said as being bad and evil because mages are placed above the common rabble.
This notion that just because mages are in positions power and therefore it is wrong...It is the equivalent of the argument "men are the majority of the ruling class, therefore it is wrong". It is the equivalent of the argument "well capitalists the majority in power, therefore its wrong"
For one, this position completely forgets that people can be representative of other people. Mages can represent non mages and vice versa. Men can represent other men and women.
A good leader of is an intelligent and well educated intellect. You seem to completely miss out on the fact that there are very few if not no educated intellectual outside the nobles and mages. Not because of "Rawr, oppression !" but because of a myriad of factors. There is no printing press, there is a lack of societal mechanization to build education centers, there is even a lack of surplus of food & labor in many parts of Thedas, the Dalish are nomads so they cannot settle down and build a civilization so that knowledge filters down to whole of society.
If we placed every group of people in Thedas side by side and viewed them using their merits, the most intelligent and intellectually capable, therefore most suited to rule will be the nobility and the mages.
Mages and nobles are the ones who are the most learned. They spend their time studying, mages even more so because it is mandatory. The magical powers need to be learned and mastered. You must control power, lest you let it control you. If you take Tevinter out of the equation, you will see that mages are in positions of power because there are no nobility among them. These societies are just substituting the nobility class with the magi class / intellectual class as rulers. There is nothing inherently wrong with that idea.
Additionally, you also seem to be forgetting that barring Tevinter, other societies where mages hold power are that way because people respect the mages. Magic is a gift of the Creators to be used. The Keepers and the Firsts are the wisest of the Clan and shall play an important role in its governance. Rivaini Seers understand the reality around them a lot better than others because of their magic.
Again, this all boils down to "Rawr, Tevintah is bayddd ! So everything else magic and mages are bayydd !" It makes no sense. Why don't you actually try to consider the alternative viewpoints instead of just looking at it and go "Powah ! Privileges ! Oppression !"
So...what exactly do you propose Cass should have done?
Even if they were monitoring the Circle for unrest from before Hawke arrived; and there wasn't really a need for it back then; she would still be unable to learn Hawke's intentions without interrogating one of the crew which is precisely what she does at DA2.
At most, she would have been able to gather what Hawle was doing, not why he was doing it.
Cassandra should have investigated and done her research. She is the Seeker of Truth, not the Seeker of Bullshit, not the Seeker of False Informations.
The fact is, she did not conduct a proper investigation. All she did was track down Varric and interrogate him. We do not see her or any of the Seekers doing an examination of evidence at the crime scene (Meredith lyrium statue), we do not see them interviewing the nobles who Hawke should be well known by now because Hawke was a Champion for a while. We do not see them investigating the people in Lowtown, where Hawke started his/her life. We did not see them going to Danarius' estate or to Anders' clinic to do investigation.
All we saw was Cassandra bullying Varric into a confession and stabbing her books. That is not how a detective is supposed to work. That is how an idiot who does not know how to investigate things work.
I have not even gone into how the Seekers of Truth, while claiming to root out Templar corruption, did not even once, in the span of the entire DA 2 game, visited the city to inquire and investigate the corruption and shitstorm that was brewing.
So excuse me if I am not impressed by the abilities or rather the lack of abilities demonstrated by Cassandra and the Seekers.