It's baseless speculation.
Not really. Unless the Qun helps overcome the nature of all people, which it doesn't, someone somewhere's going to abuse it. Though as I said, the most likely case is a dead Sten and everyone moving on.
Sten's not the only one who mentions it. Fenris and the escaped Tevinter prisoner from one of the codex entries mention that Qamek is reserved only for those that refuse to submit. And you said "he should by all accounts be qameked" as if it doesn't make sense that Bioware's story doesn't conform to your speculation. It would be ridiculously wasteful to use it on anyone else. So no, there are no accounts that suggest they would use qamek on him when he returned.
I don't care about what everyone outside of the Qun says, without a first hand account, I'm going to assume some people use it for other reasons besides what it's supposed to be used for. Does that make it likely? No. Does that mean it never happens? No.
I read this several times, and I'm still not sure exactly what you are trying to say.
Your entire argument is based off the fact that Sten is allowed to return, completely disregarding that Sten could have just as easily hid all the facts of his journey. Instead, you act as if this one singular Qunari is in the wrong, when all other sources point to him being right.