First, forgive my wall of text. Some ideas I thought I'd put to print while they're fresh.
Read Teagan's toolset notes, In Exile. Your assessment of his character (while I can see your perspective) is way off base. Teagan isn't a 'lying, manipulative hothead'. He has flaws, but those aren't among them. First among them is Teagan often defers to the expertise of others because he lacks confidence. He's the one in the best situation to deal with the undead menace in Redcliffe, but he defers to the warden. And really, if he had been more confident, nothing would have been going on except Eamon's poisoning when the player arrives. It gives the player something more to do than just 'go look in on old guy sleeping'. It showcases Connor's situation and that of young, untrained mages in general. Beyond that, yes, he didn't say anything about the tunnel leading in, but the village was in dire straits and he knew it. It was the only way to get you to help. Had he said, I have a way to get inside the keep, most players (except the most completionist) would have skipped the battle altogether and gone into the keep. Especially after doing it 10-12 runs. In this, he is an effective story/quest hook.
How awful, "he plotted to trick" your warden into saving all those lives.
Why must Teagan's reasons for calling out Loghain be only because it's a reaction to his king being dead? I read it as a reaction to both his king and his family--not just Cailan but Alistair as well--being dead. Teagan is the only Guerrin who displays genuine pleasure at Alistair's surviving Ostagar. If you don't have him in your party, he asks after news of Alistair (Did he die? Is he alive?) Teagan's second toolset notes state he will call out someone when he sees injustice being shown. Loghain doesn't just return and bully the bannorn into accepting his regency, he also doesn't give up any explanations for Ostagar, and slanders the wardens laying the blame on them for what happened. The Bannorn are still waiting a year later at the second Landsmeet for answers that never came. And no, I don't want to turn this into a discussion/debate about Loghain's behavior.
Even his stance during this confrontation is defensive, with arms folded across his armored chest, his feet shuffling from side to side and looking askance. He's not comfortable there. You can see it in his body language. I expect he fully expected Loghain's supporters to cut him down on the spot for speaking up at all. He isn't calling him a power mad traitor, he's just demanding answers to questions Loghain is side-stepping.
Family is very important to Teagan. He puts up with Isolde, loves his brother and nephew--and Alistair as a 'sort of' nephew. In a letter you find in Trespasser, Alistair even calls him "Uncle Teagan".
On topic, in the parameters of this situation, I went into Trespasser with an open mind. Okay the unfortunate combination of Loghain/Howe traits in his face made me wish for a bag to cover him up with, but Teagan is still Teagan, albeit an irascible Teagan. He's not there to be friendly with the PC. He's there to make the point across that the Inquisition needs to disband--and the events that transpire demonstrated that to my particular inquisitor in spades. The thing he said about the wardens is a reference to old history (anyone know what he says in the instance of wardens being exiled after Adamant?) And during Corypheus they were a threat. If it is common knowledge that they were controlled and part of Justinia's assassination, it is even easier to see his point of view. He isn't referring to the HoF and Alistair, but I'm pretty sure a lot of people took umbrage at his words because "warden" was used in a less than complimentary tone.
The wardens were corrupted by their very desire to protect mankind. Blackwall even says that was a crappy thing to do, to turn that desire to do good and use it for his (Corypheus') own ends. Right now, the wardens are under a PR crisis, and their secretiveness has come back to bite them on the ass. And it would hardly have done his case any good to say "Well, this happened 200 years ago, but the last couple of wardens were alright guys, so I was all cool till this happened. But of course, the Hero and Alistair resisted this problem." If you did Warden's Peak, you finish it learning that Sophia was no angel, and the wardens were used then by her as well. Something Teagan is afraid will happen again.
The other delegate, the Orlesian is all 'stumble all over himself to prove how accommodating Orlais will be' obsequiousness that frankly, I found so saccharine I thought I was going to develop diabetes if I had to listen to him much longer. Teagan hates politics (or did during Origins, there wasn't any new information in Inquisition to disavow this personality trait), his trips to Denerim involved more hunting trips with his nephew than Landsmeet concerns. His holdings in Rainesfere are small, and he deferred to Eamon on all matters of governance (he won't even assume more than managerial duties in the hopes you can revive his brother). Again, this plays into his lack of confidence.
Orlais is champing at the bit to re-annex Ferelden, and here's this mook trying to get the Inquisition into his pocket to use it's influence/martial troops to give it that much more power to grab more real estate for the Empire. I am unsure how different the situation is if you have Celene ruling. So far, with the "Gaspard rules from the grave" bug, I didn't get to see what, if anything changes.
Add to that, the Inquisition owes it's loyalty to no one. Two years of having them sitting on your doorstep, hovering like a vulture and uncertain of their motives would ruin anyone's day. There is a reason the only option to keep the Inquisition leaves it as Divine Victoria's personal force.
So, I'm going to go on the assumption that this was Teagan, having the mother of all bad days, even worse than when Redcliffe was under assault by undead. So much so, he's lost all patience dealing with it all. Not a good quality for an ambassador to have, but we all have bad days eh?
So, this is Teagan. On a really bad day. That's my story and I'm sticking to it. Bioware certainly isn't going to change it, and I can see it within the parameters of his characterization.
Just get me an Orlesian mask. It worked for the Phantom. I wonder how well Teagan can sing? 