This is all coming only from my heart... so please do not be offended as it is not intended to be meanhearted in any way. I am only sharing from my previous hurts and past as I have been there on both sides.
I am not sure what the serious 'myspace' offense was... However, you may have to realistic about this one during this day and age. (I believe there are parental controls or you could request her password to allow her back on.). Talk with her about her activities online to be more secure.
But realistically, once she is an adult, this may backfire on you. I am only sure of one thing... Severely discipling your child will result in a rift with many family members. It sounds to me as if your mother was rather hard on you in your childhood, too. I feel you grew up with the idea that you need to also enforce the rules with your children.
It also sounds like there could be a possibility that your parents changed and perhaps mellowed with age. As long as your child is still alive and no harm is done to her; I would see no problem here. You can always continue the 'grounding' when she returns. She may even return for the better depending on what occurs on the trip. She may even open up to her grandparents (depending on the relationship) and feel better and see clearer the things she did wrong when she returns.
Do you not believe in second chances? Or that people can change only for the better?
It took me six years to mend my relationship with my mother that was overly strict with me. I never spoke to her or anyone on my mother's side for that long. I do speak with her now, but there was more to the story and I still refuse to speak to the rest of the family. Would you want that for you and yours? It may happen and it may not. I am only sharing what I know from being in an overly strict environment without any communication with my parents. It makes a HUGE difference when you can openly talk with your mom and dad without consequences. Otherwise, one will want to hide the wrongdoing.
As for now, being a new mother... I am completely opposite of my mother. I try to keep my temper in check and I take a timeout from my son. I also spend all my time with him, unlike my mother. I get down and talk with him on his level. I do not want the same occurrence later in life to happen with my child.
It sounds like you are trying with the counseling, but you may need to be open with your mother and your daughter in a way that could be difficult for you. To do so is to be calm, patient and understanding... this is hard for many when we are upset.
Best to you and your family and I truly hope things work out.