It does beg the question on HOW the Seekers get a very specific type of spirit to do exactly WHAT they want it to do.
Well, I'd imagine that those they tend to recruit are extremely pious. Their faith would act as the beacon that draws a spirit of Faith in turn. Even then, I get the feeling that failure rates are high. Indeed, it seems that when a mage tried to join their order and failed, that is when they chose to apply the practice in the Circles.
But... How are the Seekers rendered Tranquil without their knowledge? If the Lord Seeker is the only one to actually know of the process at any time, is it the Lord Seeker that has to personally render each candidate Tranquil?
If they are branded with Lyrium, are the scars not permanent? I remember that the Tranquil in DA:2 had the Chantry's sun motif burned right onto their foreheads, but neither the Tranquil in DA:O or DA:I had such markings. Since DA:I ruled out a visual retcon like what happened with the Qunari, that means it was just a Kirkwall thing. Yes, that's the kind of Circle Meredith was running... and the Seekers found nothing wrong with this.