Can I suggest a slightly different approach? I used to teach college classes and often heard a similar complaint. When I got the students in office hours, they were complaining that they could not possibly complete the reading assignments they'd been given. But, the more I asked about their study habits, the more it became clear that they just did not have good study skills. Things like time management, mnemonics, fast and accurate note-taking, scanning texts for information, figuring out which information was useful and which was not, regular review of past concepts, summarizing information on their own and putting it in their own words, searching for other sources of the same information, finding books and articles other than the text, they never did any of that. They just sort of sat down, plugged away, got frustrated, and tried re-reading or re-doing the same problem over and over, then realized that they were still only on Chapter 2 and it was past bedtime.
Many primary school teachers don't have a separate class wherein they teach study skills. The high school I went to specifically had a study skills class to remediate this issue. I realize learning the Cornell Note Method probably isn't going to help your daughter, but mnemonics, starting with a 10-minute review, efficient reading techniques, and time management are good things to learn--especially for when she's older and needs to apply those skills to things like taking the SAT. If teacher is just checking that the homework was attempted, then what is the minimal attempt needed to get credit? If she is stuck on a particular concept in the reading homework, can she skip ahead and work on something else, then come back to the more difficult bit--at least get partial credit?
Can you suggest to the principal that the students entering this teacher's class maybe don't have the level of study skills she was expecting, and suggest that she needs to address this issue in class? That way, you are suggesting a constructive solution to the problem rather than putting the principal between a rock and the teacher's union, and you won't put the teacher on the defensive so much. Plus, your daughter would get extra study skills that would help her be more advanced than other classes of students next year. After all, NOTHING the 6th grade teachers can throw at her will seem so difficult compared to the Homework Lady!