Many hugs. I just skim read through your thread. I really feel for you that these NWs have been going on so long and you are obviously concerned about your DD.
I don't have a huge amount to say to help really. Only that Tracy did say it is normal for a LO to wake in the night (and that NWs can be long) and so long as she is not calling for you the best thing to do is to just let her be. I know others have advised this and you've said you cannot settle yourself when yo know she is awake, I totally understand that.
I think once you have looked at all the possibilities (which it seems you have) then there really isn't much else you can do other than accept she is resting (if not sleeping) and you have done your best for her. We can only set the scene for sleep, we cannot make it happen.
My DS also never slept 12hrs as a baby, 10.5 to 11 was his max, that's just him. We had some longer nights when he dropped to 1 nap but it didn't last all that long and eventually in the few months running up to dropping the nap all together his nights were down to 9hrs. I am very happy to say he now sleeps (no nap) a full 12 hrs at night
I realise nap drop prob seems a long way off for you.
WRT as you have tried so many things already. I would regulate morning WU (whatever time you wake her on your work days), push the set nap later and set BT a bit later, more like 8pm. Don't aim for a night longer than she is capable of. And really really do your best to leave her be when she NWs. If she can in any way sense that you are awake your wakefulness may actually be keeping her awake - in the same way that her wakefulness is keeping you awake.
I think you already mentioned language explosions, 18 months is a real time for language development and caused a lot of sleeplessness here, as have every other development.
hugs, I hope you get some rest for yourself x