The Inquisition troops are supposed to be men he brought from Kirkwall, though, aren't they? Former subordinates of his who became disillusioned with the Templars. As for it being an individual mission, I don't see why. Unless there's something important I'm missing it seems to me that Cullen's approach to getting you into a castle is no less legitimate than Leiliana's.
the whole hard choices thing. If you have an option with no negatives and a positive (sending in cullen and his troops) and an option with negatives (leliana getting tortured) no matter how good the positive people will inevitably chose the first option.
there's are the sort of "cheating" choices bioware seems to moving away from.
take the M/T war so far all indications are their is no I win button no alliance where everybody just kisses and makes up. But a hard choice where you must actually live with your decision.
whenever there's an option to have a zero negative outcome people will take it. Cullen busting down the door would either be that zero negative unless he failed as well and if not tortured the negative still needs to be on par with Leliana.
zero negatives are not a choice they are an I win button. give that the entire thing is a fundamentally shift in Lelianas character it seems very personal not a place holder scene, where you insert X character and the scene plays the same way.
Cullen for example already hates mages so the scene does what to his character? re-enforce a viewpoint he's held for over 10 years? He's also male meaning its much like the Alenko decision (he's bomb tec and is pretty sad character as well as being male) the decision does not become who do i choose? but a do i sacrifice Alenko?
this would be no different if only Leliana gets the negative outcome, you only choose it to experience it unless your RPing a ****** nobody would choose for any reason other then to see it (i win button scenario why have her tortured if you don't need to?)
and if they both get captured?
Leliana mostly likeable character in her third game, spy master and deeply emotional character (we all remember leliana's song)
Cullen Templar knight likeability split first time he's been something other then a background character, hardarsed mage hating general and male.
it is much more palatable to send him to be tortured, for those reasons. unless your RPing someone who hates leliana or is in love with cullen.
it becomes an Alenko choice where its almost too easy to choose one over the other because the outcome is the same.
but if it were solely leliana's mission to slip in and you don't do it and not sending her in results in a very negative outcome such as the loss of massive troop numbers or even a party member.
suddenly you have a duel negative if the party member dies there gone for the rest of the game but depending on your choice up until now (such as pro mage) sending leliana in may stuff up things where see despises mages so much she doesn't do certain missions wont send support etc, which has dramatic repercussions down the line.
Suddenly the double negative is not so easy, the choice is as stark as making one choice.
then there's other subtle ways of doing the same, such as having her tortured actually hardens her so at some later point in the game she doesn't hesitate and she saves herself/someone else.
whatever the case there has to be a reason a hook that makes you decide to send her in it has to be a real choice, or its the Q/G war all over again where there's a big flashing i win button.