Why is Tranquility only ever presented as a thing for Mages?

You can’t equip armor on companions in DA2.

I thought the physical branding was separate from the one used to inflict Tranquility. It was never used in Origins, at least, or was that another "just pretend all the non-Sten Qunari have horns" thing?

I'm not sure it's that they can't fight, as much as they don't have an innate sense of self-preservation. Basically, if you try to kill one, they won't fight back, but if you were to compel to commit an act of violence, they'd still do so to the best of their ability, like with Maddox in Samson's quest. Even taking out the combat aspect, though, Minaeve also mentions Tranquil can focus on their work like no one else can, and even outside of military life, I could see some people accepting Tranquility as a fair tradeoff to master their craft.

Hmm, the idea that it would be harder to sell conveniently as an "easy out" for Mage problems, I guess, could give some justification for why it isn't therefore used in other places.

Yeah, it's similar, but different, I think, based on how its described. Tranquility explicitly retains the mind, while sacrificing the "heart", if you will. It's very different from a lobotomy, as Reznore suggested. The Tranquil remember their lives before, still have all the same knowledge and ability they had before (sans magic). But Qamek, according to Bull, IIRC, leaves the person pretty much empty mentally. They can perform menial tasks, but their IQ reduced to an animal state as well. Nothing about the individual or the individual's mind remains.

I play on console so...

Lol. So you would allow a young man to be possessed by a sloth demon, just to stick it to Anders?

Lol. So you would allow a young man to be possessed by a sloth demon, just to stick it to Anders?

We attack them, fight them like we would kill someone else. We certainly didn't do what someone would normally do to "wake up" 
Pretty sure the reason it's different for Fenyriel is the fact that he's a Dreamer. I'm pretty sure that's what Marethari says before the quest starts. 
And no, I don't think Justice slows down the taint. Grey Wardens live around thirty years. At most, after Anders's joining, it's ten. 

There's nothing that hints at that, is there? Well unless you assume it's literally impossible to turn a non-mage Tranquil, and I don't think that was ever explicitly stated or strongly hinted at either.

It also wouldn't make much sense given that Cassandra has no idea she was made Tranquil until she reads it in the Lord Seeker's tome, and if she'd had magical powers that suddenly went from normal to much more muted, you'd think she'd have suspected. Not to mention that, as far as I've ever heard, Lambert speaks of mages as though they were a group he isn't and never was part of.