Just seen this thread (as I often seem too busy to properly catch up with anything online!). We've had nappy issues with Clement lately - rashes and not really sure what works for him. Sometimes I think he seems to develop sensitivites to fabrics (whether fleece or paper liners) which he didn't have before, which really confuses me!
We used fleece for months since he was born with no problems - but the he started turning red when wet fleece was next to him! And then it seemed that paper liners could do it, esp if I'd washed & re-used the unsoiled ones!
Anyway, wondered if you've tried a Swaddlebees one-sized cotton velour nappy? Though if you say she is a heavy wetter, not sure how it would work for that. But I think it must be the slimmest between the legs that I've ever tried. It fits almost as neatly as a pair of knickers! LOL And the elastic is very un-tense, so seems very flexible and comfy, I'd have thought. And folds down neatly if you want it lower in the rise (since it's btp). But I think I felt it was the best one-sized nappy for a newborn, as there was *NO bulk between the legs (like you get with most other btp nappies!), and I loved it on Clement as a newborn.
Just an idea, but so sorry you've got such problems with your little one!