Sounds like you may be willing to give this girl a free pass if she doesn't prefer the expander and if she may go into a rage if it's uncomfortable in any way. If this is the case, you may want to just let her teeth go bad. At 8 years old, she is already learning that she should tolerate no inconveniences. If she can't "stand" and palate expander, what will happen to her if something worse happens one day?
If I complained about my clothes etc at that age, my mom would have taken me to the thrift store to donate them all AFTER some other firm punishments. My brother and I knew that any complaining usually lead to volunteer work over the weekends at wildlife centers or food banks (actually it wasn't because we were complaining, it was just what we did), and we were always very aware of the suffering of other kids.
The issue here is not the side effects of the expander. Of course it will be annoying, as will many many things in life. I had very painful braces as a child, and aside form legitimate pain, very little complaining was allowed. I was reminded how lucky I was to live in a country where I could have the opportunity to fix my crooked teeth.
Show your daughter some footage of needy children with cleft palates who cant' get them repaired etc and tell her she is fortunate to have dental services. Tell her she will not be allowed temper tantrums or "x" will happen and follow through. Either that or tell her she can skip the palate expander and address her crooked teeth later as an adult with her own money.
Be firm! Good luck.