Sorry, they're ALL valid as a way to learn about Tevinter. The primary reason I only pointed out those three is because those are the only people you can have involved conversations with about slavery in Tevinter, and they aren't standard NPCs.
Yes, the institution of slavery is a terrible, horrendous thing. But not all masters are the same, nor do they treat their slaves the same. Likewise, not all slaves are treated the same or have the same life experiences. There are probably thousands that do physical labor and aren't in a position where they are at risk of being scarified for some blood ritual, and some of those, depending on the owner, will be worked to death just like those in Kirkwall's Bone Pit. There are also probably literate slaves whose sole function is to copy out books. Tevinter is a very large nation and they need millions of bodies to keep it running. There will be menial tasks, humiliating tasks, degrading tasks, simple but repetitive and mindless tasks, labor intensive tasks, dangerous and deadly tasks -- tasks of all kinds. There are so many millions of slaves in Tevinter that the number that are bled in some form is probably very low. Low too will probably be the number of personal slaves, such as the one from the codex entry.
Many players, primarily those from the United States, will hear "slavery" and immediately associate it with how it is viewed in the US, which elicits an immediate and visceral reaction, which is perfectly understandable. But throughout history slavery has had many forms, including as someone pointed out earlier in the thread, indentured servitude, but I guarantee that most of those same players would not consider Hawke to have ever been a "slave."
All of the different perspectives are important because they allow the player to take in various accounts and form their own opinion. Dorian even admits that he is now thinking about slavery in a different way since being in the South. His challenging the player does not make him a "slavery apologist" as I've seen him referred to elsewhere, but only that he's trying to get the PC to look at it in a different way and think critically. Out of all the DAI origins, the non-mage Trevelyan PC -- which mine is -- is the most hypocritical for his superior attitude about it, which Dorian rightly points out (in a more tactful manner).
But as I've said elsewhere, the problem with the Dorian slavery conversation is that it really isn't involved enough and doesn't allow the player to offer more direct challenges that aren't based on simple outrage, leading to a pattern of accusation and defense that is not productive or conducive to learning.
But back to the topic... If they are going to focus on the downfall of Tevinter it does present an interesting opportunity to have the player start out as a slave, but I'm not sure how that would work out logistically for Qunari and dwarves. If they managed to do that, it would be the first time in the series that the human wasn't noble in some form or other, which would be refreshing, as much as I'd love to play an altus and see everything from that perspective, for both mage and non-mage classes.
It would be nice to have the PC be a sort of hero-type that was basically a normal person that stepped up and showed a willingness to do something, similar to how the PC was portrayed in Neverwinter Nights. In that game you were basically just a tough, competent person, but you still took orders from higher-ups rather than becoming the leader.
The only downside about a Tevinter-centric game for me would be more Qunari involvement, which I'm sure would be a source of conflict, particularly given the remarks at the end of Trespasser. I'm not too keen on the Qunari in general. Of course, I'm not too keen on elves or elven lore either, but we have the whole Solas thing to resolve. I'm sure I'll muddle through somehow.