Unfortunately split or composite classes are never ideal. Not for the kids, teachers, Principals or parents. There is no "right" way of splitting a class and no matter how you do it there will always be issues or concerns.
I agree that talking with your Principal and teacher is the best thing to do. And doing so as a group will mean that you all will come out with the same information. Definitely discuss the concern of the 5 girls not being included - surely things like sport, excursions, where they sit for assembly etc even things in the afternoon sessions like art, music ....
Make sure you go in with specific questions but remember to try and keep the emotion out of it - extremely hard I know. No one has put your child in this class because they don't like her, she can't learn or they just pulled names out of a hat. There would have been a lot of consideration of deciding who would go in - and believe me no matter how long you take you never get it right - there is no ideal! Try to go into the meeting thinking you can support the school in making this work!
In terms of talk about the teacher - like any organisation some people are better than their job than others and this is no different in education. If you have any concerns go and see the teacher or Principal straight away so the concern or issue can be sorted out quickly.
Kids shouldn't need to do more than 20mins of homework a night (particularly in Primary School - though this does vary as they get towards the end of Primary School). Actually to be honest research has proven that completing homework actually has very little to do with academic achievement - now what can of worms that statement might open.
You sound like you are a very supportive parent of both your daughter and your school - which is so nice to see. That is more than a lot of children will ever have!