From the Dragon Age Facebook page

I agree... I think she definitely has a fierce/dark side, and can be destructive and vengeful when it comes to her nudging and protecting of the world..her help often comes with a price. But I'm not convinced she's going to just stand by while Solas goes totally nuclear on Thedas ...if she is somehow still alive, or if not, that Morrigan would.
You folks realize she actually had a hand in creating what the elves had before Solas destroyed it, right? Or at least, Mythal did. Flemeth might have shaped the current world, but it seems to me that like Morrigan, she has a vested interest in preserving what she can of the wonders that still remain.
So she already wasn't happy with Solas. He already destroyed the world she created. If he goes through with his plan, she will presumably be even less happy.
Or will she?
She keeps mentioning revenge, she told Lavellan (who asks why she's not helping the elves) that "You know not what you ask."
She's not necessarily opposed to restoring the world to its original state. Remember that Mythal had a hand in the creation of that world, or at least the people who populated it. Her interest in preserving what's left behind may also be a huge clue about that.
I'm just going to put it out there that I'm 100% sure she's not a holy paragon of all that is right and good (there are some fans who think so), but she's certainly not 100% bad either. She's always been helpful to our heroes, but there have been many many hints that Flemeth has a serious dark side. It seems that more than body-snatching was mentioned in her grimoire and whatever she read, Morrigan found it disturbing.
It seems hard to imagine why mage!Lavellan would be sent to the Conclave.
Spying is closer to a hunter's expertise, as what they do involves stealth and investigating surrounding areas. They are also the ones who venture into human villages to trade with them. In short, most of the clan's field work rests on the hunters' shoulders.
The First is... well, the bookworm that sits around all day studying magic and history. And s/he is likely too valuable to be sent into a risky task unless his/her presence is absolutely vital.
Not really sure why is spying supposed to be a domain of hunters? Hunters don't spy per se, and the spying at Conclave couldn't be anywhere close to cartoonish walking on tiptoes and hiding behind pillars as if they're hunting an animal prey.
Plus, hey, look at real life - in spy movies we need all those top agents and advanced tech, while in reality the Panama Papers were carried out causally on a cheap memory disc
All they need is not bring too much attention to themselves, which is generally what most elves are pretty good at.
Anyway, I'd say that mages, and especially Firsts would be suited for spy missions as well as others, if not more - they usually take care of things like dealing or contacting strangers and know something about outside customs or politics (might use it to soothe out nerves or be less conspicuous), probably know how to read and write better than most (correspondence and interesting documents) and even have a legit reason to be at the Conclave, unlike random non-mage elves with Dalish tattoos - after all, they're mages, and the Conclave is in large part about mages.
Is it sensible to roleplay that Rogue Lavellan and Warrior Lavellan did receive some education from the Keeper or from their older peers ? It is sensible to assume that non-mage Lavellan know to converse fluently in Common, know the basics of Human history, know what the Dalish about Elves (even though many of it is wrong) ?
I ask this because I find it really hard that Deshanna would send away one of the Rogue or Warrior of her Clan off to a large conglomeration of Humans without preparing them for it. The only real clue we have on how Rogues and Warriors are treated by the Dalish Elves is in Dragon Age Origins. In Origins, Mahariel could read the books and Codex entries around the camp. Tamlen is also familiar with some Dalish lore. However, Mahariel was not as knowledgeable as Merrill was.
Thoughts ?
Going to weigh in on this argument... the First and Second would be the best-educated people out of necessity. And they'd have to be excellent at avoiding notice due to that whole "we don't want to be murdered by templars" issue they have. Also, a First is someone explicitly trusted by the Keeper; otherwise they would not be First.
Although the rogues and warriors might not be as well educated in Dalish matters as the First is, they also make good choices to send out because they're certainly used to going out and trading/raiding/etc. against humans.
So basically any of them is a valid choice...
But there's also the possibility no one here might be considering. Although undoubtedly Deshanna is interested in the outcome of the Conclave (and likely, she does NOT want the Templars to win as it will make things worse for the Dalish), she could be sending someone away who is inconvenient to have close by. The First (or hunter) could be particularly unsociable, or have done something indiscreet, or maybe they're incompetent at their normal duties (but decent at being nondescript). It also could be a test for a given person. Maybe the hunter wants to get married, and this is the task that is set--return safely with information. Maybe the First was only recently given that position and the Keeper wants to be sure he/she can be trusted. Or maybe it's a general test of ability.
No one in a Dalish clan is expendable. They have fewer children every year, entire clans are always being wiped out, and probably each clan doesn't consist of a large number of people. So, each of those people need to work to sustain the clan, regardless of their role. So... no, a rogue or warrior is not less expendable than a mage. In fact, if there are extra mages, the mage may be more expendable (bearing in mind that they're still not actually disposable personnel--if not this clan, another could use another mage), assuming we agree with what Vivienne tells us about our characters' own culture which our Lavellans likely know more about than Vivienne (and there wasn't ever that numerical restriction before, but whatever).
Anyway. It's late and I'm not even sure if what I said just now made sense... so...