How can you be so sure Lamber was a pawn put in place by that mysterious opponent? I suppose you're referring to the upcoming villain in Inquisition, aren't you? But Seekers are chosen by the Divine.
However, now that you mention it, there is a mysterious event that kickstarted Asunder's fallout: the assassination attempt on the Divine. Lambert never found out the culprits behind it. Wynne guessed there had to be more people involved for a mage to reach that far, maybe even templars. But all of them were distracted by many other things.
That means that the Divine might be very interested in helping the Inquiitor to find out the truth.
My personal opinion is that the Inquisition will become and equivalent of the Grey Wardens in the end. The story of the origins and rise of the Grey Wardens during the First Blight mirrors DA:I very well.
Yes and no. I was referring both to the "big bad" in Inquisition, but also more directly to the players Justinia can combat. First, I do think there is evidence (the end of DotS and in DA2) to suggest that the Divine (and the previous Divine) have some forewarning about the events of Inquisition. But the only way they can combat it is to try to stabilise the land as much as possible, while the big bad seems to be trying to do the opposite. There's no direct evidence to show when Lambert was made a Seeker, or promoted to Lord Seeker. But I believe he became one (or at the very least was promoted to Lord Seeker) some time after the events of DotS when Divine Beatrix III became senile and more hard-lined Chantry insiders ruled while she served as a figurehead. I don't think it makes sense for a sane person without an agenda to put someone so obviously biased against mages at the head of an organisation whose purpose is to police Templars.
As far as who she is actually opposing (because I don't think she can directly fight the big bad), the implication I got from various passages in Asunder is that the ones who benefit most from instability within the Circle and the Chantry are the Chantry members opposed to Justinia (as Wynne mentions a couple of times) and Gaspard de Chalons, who benefits from Val Royeux being unstable during Celene's absense. De Chalons is also apparently against Celene's more diplomatic approach to dealing with mages and elves, so he would find friends amongst both the Chantry and Templars.
I wonder if Lambert isn't actually the one behind that.
A well known White Spire Libertarian just happen to show up at the one ball Justinia happens to grace with her presence? Shortly thereafter Lambert ousts the elderly Knight-commander (knight-vigilant?) and takes control over the largest tempar force in western Thedas despite being a Seeker?
Wynne has an awful good point about it being suspicious. There's no proof... but it fits. And he certainly seems ruthless enough to actually try something like that.
I keep going back and forth between believing he was merely a pawn and wondering if he was complicit. His dislike of the Divine is pretty obvious from the beginning. And he doesn't seem interested in investigating the Templars' role in the assassination attempt. He just wants to pin it on Rhys' fraternity and wash his hands of the matter. But is it because he's blind, or is it because he already knows who was responsible?
Either way, the same ones who benefit from Justinia's assassination attempt benefit from Lambert being in charge of the White Spire. But the forces behind it benefit the most, with the players putting a spark in charge of a powder keg.