Firstly I think we need to address what constitutes a shoujo-ai relationship. The definitions do vary but I'll stick to the simple a woman being in love with another woman. In that case, the answer to the question will be yes or no depending how you define love but to leave it at that will be truly unsatisfying. It is the nature of the relationship we really want to discuss here.
I think the important question is what characteristics of Mireille and Kirika’s relationship has so moved us?
But first I like to digress on some old input
Kirika, meanwhile, is experiencing a far greater degree of separation from society. She is completely and utterly alone - and, unlike Mireille, she has a burning desire for human friendship. Indeed, Kirika is separated from humanity - as I have mentioned before, Kirika is like the robot who wants to become human and can't. Yes, Kirika is human; but all her humanity takes second place to her Noirness. Her desire for normalcy is the weakness in Altena's view, and it is tied (I believe) with her habit of not killing till the last possible moment. She has to look her opponents in the eye - not to feel their soul, but to try and catch a glimpse of her own reflection. Think of the amnesia sequence - I am Noir - finds ID card - slightly shocked to find picture on ID is self. Her attachment to her ID card - made for humans by humans; her one possession that points to Kirika Yuumura, and not Noir.
Kirika's attachment to Mireille is a natural impulse qua Noir; a desire to seek those of one's own kind. But more than this, it is a natural impulse to seek love in that which is not only similar in the Noir level but is patently human - there is nothing robotic or inhuman about Mireille. Mireille is communicative, passionate, selfish, a bit foolhardy - the essence of the human being, in a way. Kirika feels hints of emotion but can't process or express them; Mireille does this on a daily basis, she speaks for them both - "futatsu". Mireille, in other words, is Kirika's link to humanity.
I think Kirika’s lack of exterior and conscious emotion is not indicative of the fullness of her interior emotion and subconsciousness which I think runs quite deep. For example her tears in episode 1 is a reaction from a part of her humanity she cannot comprehend. We agree Her Noirness so to speak is not the only intrinsic part of her being but I think that humanity is a lot stronger. However her amnesia and complete alienation to humanity has made this abstract concept Noir and herself the external assassin black box (pun sort of intended) the only concepts of her own existence and experience her conscious mind understands. When she gazes with those eyes whether out the window or into a cup of coffee she is trying to communicate with the hidden humanity of Kirika. This also gets manifested in various ways such as the instinct to pick up Rosalie’s orange out of kindness, the feeling to protect Prince Myshkin and her affinity with Milosh. When she kills I think there are subconscious reflexes which numb her very strongly.
As you say Mireille is a character with vast independence, a strong sense of mistrust and highly repressed memories. Perhaps this is what allows Kirika to feel for her, they are at a subconscious level birds of a feather repressed in their outward expression and intimacy, traumatised by a past they can’t quite grasp. Mireille’s repression is a more conscious one, She has hidden herself beneath the assassin black box willingly. Agreed Mireille’s humanity is not so hidden, her biases and showing of temperament is far more clear, she can be rash, she can be jealous, she has her whims and opinions.
My point is, Kirika becoming Kirika is reason enough for "Season of Hell", and reason enough for the passion of the K/M relationship. I don't mind calling it passion, but I do mind calling it eros...but as I remarked on my website, the argument as follows works either way. I.e., I would posit that Kirika's inhibition to becoming a human being isn't her lack of sexual activity - though even if it was, she has got a far greater obstacle. Put yourself in the mind of Noir - the mind of a being whose duty and vocation is to kill. What's your problem? You can't feel sad. You can't feel remorse. You can't accept the fact that you've done something very very bad, so you can't develop, you can't be human.
The lack of aversion to killing may not be a lack of moral outrage. The concept of killing another human with negative connotations as a universal concept requires some sort of implication that all human beings are better alive. This is something people often profess but arguably may not follow in practice. Mireille certainly doesn’t share that sentiment as her reluctance to kill Kirika and other characters such as Milosh, Nazarov and Claude show compared to her disdain for the men who are 'scum' in episode two. That being said, the process of killing in Noir has allusions to inner violence as well as external violence which can potentially dehumanise.
I want to highlight observing the method of expressing love alone might not allow one to accurately evaluate the quality of the love itself. Most of us here are audiovisual or cerebral individuals we might have preconceptions of love most in tune with our dispositions. A kinaesthetic person might find a certain type of physical expression the most authentic and genuine and an intellectual platonic relationship rather cold for example. I don't think whether Kirika/Mireille's method of expression necessarily allows us to discern the quality of love but rather highlight it.
But what really makes us feel somehow the relationship between Mireille and Kirika is special vis a vis other relationships? Their combination of opposites and physical attributes might be appealing and cute but many pairings are likewise. Their devotion to each other maybe remarkable but it’s not the devotion itself. One might be able to argue about Chloe’s devotion to Kirika is equally powerful. Is it because this devotion is so mutual? This may be arguably necessary but not necessarily sufficient. But this goes beyond just mutual infatuation or admiration which some might dismiss as naive.
The Soldat/Altena definition of Noir the two maidens who combine into one supreme blade might yield an insight. Noir perhaps is the combination of two who become more than a sum of their parts and wield their blade upon the Earth. We do feel Mireille and Kirika are more than the sum of their parts. There is a certain coherence between people who seem diametrically opposed and in certain areas psychologically lacking. And unlike the blade of Noir past which existed to exact judgement on all the wicked sinners; although they still kill they exact judgment only on the sinners in themselves. Their judgement on each other is a due process of forgiveness and acceptance to each other and their sins (as assassins, as Dark Kirika, trying to kill each other and their family) in a world where these things don’t seem to exist. Even greater is their judgement has been tested by trials when it was easier to let the other descend into their personal abyss but yet they persist to sacrifice themselves. So perhaps it is this genuine authenticity and intimacy required for such a process of mutual unconditional love many yearn for; tested under harsh conditions between characters flawed in many ways like ourselves.
In essence Kirika and Mireille have become The Noir within the Noir. (just a poetic way to end the post)