Homework everyday for a first grader is excessive. I'm almost certain her behavior is because she is overwhelmed. Is it actual homework she's doing or is it material she's not finishing in class? It might be possible that the teacher does not have a handle on the class and sends home the work that should have been done during class time. That could explain the reason she doesn't understand, the teacher is not effectively explaining the concepts.
I have a first grader too, so if he were in this situation I would do 2 things.
1. Make an appointment for yourself & her separately with the school counselor. You need to relate the behaviors you've observed and what has happened when you've talked to the teacher. And for your daughter, she might be more willing to open up and talk to someone who's not Mom & Dad. Once the counselor has seen both points of view, she should be able to determine what the real problem is. It might be as simple as switching teachers.
2. I would speak to the principal about your concerns about the teacher and the amount of homework she receives. Try running a copy of the weeks worth of homework so you have actual proof. The principal may not agree with you - I find a lot of educators give you that "it's not our fault, they need to know this for the TAKS" excuse. At least this way, you've gone on record with your concerns.
Here's some food for thought. As I mentioned before, my son is a first grader, like you, he's my first of 3. I had him enrolled at Timber Creek Elementary in Flower Mound for Kindergarten. The year he started, the school had gone from an Exemplary rating to an Acceptable rating. I noticed that in Kindergarten - 5 & 6 year olds had homework due every Friday and as an advanced reader, he had to read a book each week and write a couple of sentences describing it. In mid-year, we moved to Plano and he attended Hedgecoxe Elementary. The new school was rated Exemplary and there was only the occasional homework work sheet. So I guess my point is that his old school was probably trying to compensate for their low rating by starting a rigorous curriculum for the youngest students so that by the time they reach 3rd grade & start TAKS, they can perform well.
Also, in 1st grade right now, my son gets a short chapter book. He has 2 weeks to read it and a small packet of worksheets to turn in on Friday of the 2nd week. On occasion, he'll have the odd 1 page work sheet to finish during the week. That's it for homework. We pace him by having him read a couple of chapters a night the first week and when she's done a page or two of worksheet a night on the second week. A couple of times he has brought home some work that he didn't finish in class or he had to re-do due to messy handwriting, but it's rare.
I hope this helps a bit.
K.