Hugs Paula {{{}}} You are NOT a failure ... you are looking for answers and not just giving up ... that makes you a super loving mummy!!
We are having NWs at around 3,4,5am which last a few hours. but luckily Artemis is content playing and laughing in the pitch dark - but I can't sleep if I can hear her, so I lay awake and I'm knackered now! She does still have a nap - we are doing it later than before, shorter amount and an earlier bedtime to allow her time to play up or if she sleeps right away it gives her catch up sleep to try and stop the OT cycle.
We are dosing her up with paracetamol as she is dribbling a bit and gringing her teeth, so 2nd yr molars are definately moving - but no visible sign yet. Teething has a LOT to answer for!! Grrr!!!
I find that if I explain what we are going to do the next day AFTER SHE HAS SLEPT ALL NIGHT LONG, this sometimes helps reduce the NWs (but not always). So no real help there I'm afraid.
I can't remember the details for her move to the BGB while she was young, but have you tried telling her that if she doesn't stay in her bed and behave like a big girl, then you and daddy (keeping it together as a team here) will have no choice but to put her in her cot again. I don't like these ultimations much, but if she can understand that there will be a consequence it may stop her
Just a thought?
Artemis is also an extremely light sleeper (gets it from me unfortunately)
I (personally) would be wary about putting her in with her brother for all your reasons before mentioned, but also if she is being stubborn, manipulative and testing her limitations, then you will be kind of letting her call the shots for no reason other than for her to win iyswim.
Anyway, not much use here I'm afraid, but I can send you heaps of sleep vibes (you and DD) and lots of Mummy hugs