Incidents of non-mage possession are very rare, add to that the templars are taught Seekers are incorruptible and cannot be possessed by demons.
So short of having the gift of prophecy or the most extreme case of crippling paranoia imaginable. Explain how they could reasonably be expected to be on the look out for such things as the incorruptible Lord Seeker purposefully allowing one of the rarest most elusive type of demon known to impersonate him?
Oh wait you can't. This is just a ridiculous attempt to distract from Fiona's own incompetence.
Only the most senior templars can detect Envy demons, there were none of these senior templars left. They literally had no means of detecting it. Simple no? Isn't great when things are exactly what it says on the tin?
As for the specifics of Lucius' behaviour change. That could be chalked up to any number of things such as stress, the Chantry's betrayal, fanaticism or fact the sky exploded itself a new anus. What remains however, is that until "Lucius" played his hand and revealed he was Envy they had no means of detecting that was what he was
Non-Mages can be possessed just as easily as mages, it's just that they are rarely ever sought after as hosts.
Furthermore, while the Seekers are supposedly "incorruptable", it was also said that Tranquility was irreversible with just as much certainty... so that the Templars know that to be false (even if they don't know the specifics of the actual cure) might mean that a lot of old notions are now in doubt?
That the Templars often seem to be the most paranoid people in Thedas has never stopped them before of accusing mages and even their fellow Templars of consorting with demons before? In DA2, we saw Cullen was smart enough to recognise that Wilmod was possessed and trick him into revealing himself, showing that such an eventuality was on his mind, while when Hawke can accuse Ser Conrad of consorting with "The Great Demon", even this obvious lie is treated as serious enough warrant some kind of investigation.
As for the Senior members being able to detect it, I'm calling bull, you don't get better at detecting them just because you're higher up in the food chain. The Senior Templars might have more experience, but they are all trained from the same manual.
We see that when the recruits and lower ranks realise they're dealing with an Envy Demon they are able to immediately lock down on it's weakness. They might not have a detector, but all of them are trained to be able to detect them... and that they didn't was an unfortunately oversight on their part, but an oversight nonetheless.
Slightly off-point, but now you raised it, if we're talking about stress, the Chantry's betrayal, fanatics in the ranks and the sky exploding itself a new backside as an excuse for certain character flaws, doesn't that equally apply to Fiona and show that her "incompetence" is really just because she's overworked and dealing with way too much on her plate?
Okay wasn't she offered safe harbor in Ferelden? She was no longer in the circle? The crime she committed is against the King/Queen of Ferelden and the Arl of Redcliffe.
She brought in a Tevinter Magister who took over the Arl's land and people. That could easily be grounds for troops coming in. She had allied herself with a Tevinter who took lands by force.
Even if she didn't become a citizen of Ferelden, she is there under their laws.
Technically, she sold the mages into slavery when she aligned herself with Tevinter. She is living in Ferelden. We know Ferelden doesn't support slavery. The mages were never tevinter citizens, and were never slaves. She had no right to sell them.
Foreigners can't decide which laws they intend to follow.
She negotiated for citizenship and she got it.
No she didn't. She helped Tevinter take over a part of Ferelden and turned it into a slaver location.
Fiona wanted to get the rebellion out of Ferelden to stop encroaching on Ferelden's hospitality, the notion that she allied with Alexius to turn Redcliffe into "Little Tevinter" and a slaving operation is insane? The Venatori seized control of Redcliffe out from under her and it's implied that was when the Arl got kicked out as well, as a result of Alexius taking command. The Mages were just caught in the middle of coup detat, they didn't betray Ferelden in any way, shape or form?
Even Alexius states that the deal was for full citizenship as members of the Imperium, not enslavement. He explains quite clearly that while the terms of the deal did require ten years of service, they were citizens of the Imperium, who'd gain full citizenship upon the expiration of the contract.
(This is also why Josephine says we cannot march on Redcliffe in force, as politically we're seen as an "Orlesian" Chantry organisation, with our operation in Ferelden already being difficult without marching on one of their villages, to combat Tevinters and newly created citizens of the Imperium. Hence the line about how Alexius has outplayed us.)
Forgive me, but that's not selling them into slavery, that's negotiating for her people to gain political asylum in a foreign power that would accept them, rather than rely on the luke-warm hospitality of a nation that might decide not to protect them if the Templars or Chantry put enough pressure on them?
If seeking asylum or wanting to leave a country is that bad, then expats and immigrants should be hung, drawn and quartered for "treason" as well, right?
Or would we really say that people who are gay and come from conservative nations that where homosexuality is illegal and punishable by death, should remain in their native land and be killed, rather than commit "treason" by seeking refuge in countries that would allow them to live freely with full rights?
Because it's no different from the situation and dilemma the mages are facing in Thedas at this point.
Furthermore is the deal any different from what we do in our world? Seems to me no different from giving someone a work visa and making them "serve" the foreign country they're staying in for the duration of that term... and eventually allowing them to apply for citizenship as a reward?