Does make me think about DI, but does she drink excessively?
Sometimes yes, sometimes no... there are times she just sits and drinks forever and other days where I dont notice her drink much at all.
Incontinence? Although that wouldn't account for too much urine...
The peeing in her sleep was called "overflow incontinence" by one vet, basically her bladder is so full it just starts coming out but like you said that doesnt account for WHY there is so much in there... this is likely the cause of peeing in her sleep though because if it was ectopic ureter or something like that it would happen almost every time she laid down (from what I have been told/ researched)
Urine problems are really frustrating.
tell me about it 
I'm guessing it has been cultured, right?
Yes, it has been cultured sent out many times, as recently as 3 weeks ago and there is never a sign of any infection.
Ultrasound ruled out stones?
yes.