Exactly Pip - the Tonkers/Lofty dynamic is the most complex one within the squad that's all.
If there's one common theme for MR it is about love in adversity, specifically in terms of war and in Borogravia
everyone is fighting on all fronts in essence and whether or not they're in the Army or have been or will be.
The only other issue is
care in the Jackrum sense for her recruits and then for the fledgling squad, but all the girls care and love for each other in common and have more intense loving/caring relationships from the very superficial - Maladicta for her coffee; to the very profound/insane - Maladicta and her coffee...
With the squad aside from Jackrum you have that quintessential war theme of 'put 'em through hell and they'll turn out soldiers' but more importantly friends ,who work together and support each other. This is the real core of Lofty and Tonkers bonding - they have
already gone through hell, as has the other workhouse girl whose name has completely deserted me, but you know I mean...
They have been virtually destroyed by all kinds of abuse and so Tonkers and Lofty are more than lovers effectively - they are co-dependent to a very high degree where gender doesn't matter and it wouldn't surprise me if they weren't physical lovers at all. Very often sexual abuse means a total withdrawal of an individual personality to the point that any physical interaction with any
THING is utterly meaningless - not even abhorrent.
For the other poor little girl, she has retreated into madness and hallucination with the Duchess as a vaguely sympathetic but just as dysfunctional and needy alternative 'reality' into which she's retreated so she has some frail, but essentially sustainable cocoon to preserve what sanity and vitality she has left to her. Similarly, when we first see Tonkers and Lofty they are death-seeking essentially, truly disturbed and bonded to each other so tightly that they're sharing a personality and nourishing each other literally to survive, knowing there's nowhere else they can go.
I've never liked MR and I never will, because of this deconstruction of a nations total disintegration in almost every respect. It's a truly great and insightful book and mercifully indistinct in places. Through work I've seen demolished people like those 3 girls and so it's totally possible. Tonkers loves Lofty and Lofty loves Tonkers any way they're capable of, in essence because they're all they have.