I reckon it's slavery. A romanticised form of slavery, where one believes complete submission to the gods is a virtuous sacrifice, but slavery nonetheless. In some cases I imagine it may be more subtle: likely, being a sentinel requires a large amount of training, and thus slaves would likely be chosen from birth to train in those roles, expecting nothing else from their lives. It's not too different from training up an apprentice except you lack the choice between going into your trade and dire poverty. Amongst slaves these roles would likely be treasured and they would be viewed very positively by followers of those gods. I don't think it's too far-fetched a notion for people to romanticise being a well-placed slave: many modern slaves who had high-ranking positions - say, a steward or a housekeeper in a plantation in the American South - valued their roles, were thankful for their place, and internalised a lot of racist and classist bullshit in the process.
It's also quite possible that very devout non-slaves may even choose to their own slavery. It's quite possible high-ranking members of society may even be rewarded more so for this decision: they may become priests or high priests, and are placed higher up the theocratic ranks. It could potentially grant favour on a noble house from a god.
The gaes is a weird issue because Mythal seems to value choice a lot, so putting someone under a compulsion seems contradictory? Perhaps then, rather than forcing her servants to act against their will and turning into slaves, the compulsion merely reminds them of the vows or promises they made, thus strengthening and reinforcing their will to endure and continue their task.
Lots of potential reasons for this include:
- Slavery being completely normalised in the Elvehen empire. Mythal may just not see ownership, as an idea, being a problem.
- Mythal perceiving herself as a decent owner: in societies with slaves owners often make a distinction between good/bad owners to justify owning other human beings. I reckon Mythal is smart enough to see this false dichotomy for what it is: she may likely not care as long as nobody's abused (i.e. see point #1).
- Flemeth is fairly practical, and puts her survival as priority, as shown by her actions in DA:O and DA2. Likely she knows that she'd be at a severe disadvantage in the pantheon if she doesn't own slaves too. She shares Morrigan's POV and some when it comes to putting survival first and not giving a **** when it's completely necessary (although is arguably far more subtle and devious about it).
- Following the above, Flemeth likely uses slaves who are devout worshippers and willing to submit. Attract more flies with honey, after all.
- Mythal may have changed her mind over the years, especially after the empire fell apart. She and Fen'Harel appear to be on good terms (even if the ambiguity of the scene may imply underhand tensions), likely meaning she may have shifted her view on slavery. Solas gives away in his dialogues with Sera that he knows firsthand how to take down power hierarchies via. revolutionary means: it's unlikely they'd be on seemingly good terms if she wasn't at least sympathetic. Solas is pretty upfront with most people he disagrees with, after all.
- Flemythal lacking concrete power meaning she has to rely on the decisions of others to 'nudge' history. Likely this means she has to appeal to them to encourage them to make the right choices. Probably by appealing to the idea that it is their choice to save the world, as well as their sense of duty, etc.
Also, from my dialogue with Flemythal, I always got the feeling she was more interested in the idea of fate than our characters' choices: 'Luck is another word for fate'. Maybe she's just BioWare's invisible mouthpiece for why retconning every important character death is necessary. Because Fate.