Hi! Sorry that things are so hard right now.
We just finished transitioning from bottle to cup, and it took a while since it was a slow transition. First, my little guy seemed to drink much more out of a straw sippy. BUT regardless of the type, he took the name literally - only took sips. I was discouraged, and posted a question like your a couple months ago, and was told just to allow him time to get used to the sippy.
I dropped one bottle at a time, and he VERY slowly made up for the lost ounces with the sippy. I found that he does much better if I give him a sippy while he is distracted (like watching Baby Einstein) rather than at meals, when it gets tossed right away. We now have the bedtime bottle only, which I plan to keep for a while.
I don't know if others will agree, but I would say that the most important thing right now is her hydration. If she is getting less than maybe 20 oz of total fluid a day, I would give back at least one bottle. I completely understand your reasoning that cold turkey might be better in the long run, but it seems she is just not ready yet. After one bottle is cut, she might increase her ounces out of a sippy over the course of few weeks.
I hope that things get better soon. Let us know how it goes!