You have a right to use self defense when you are being attacked, whether by a minor or another adult. You can only use necessary force to stop the attack. It sounds like that's just what you did. If you get charged with a crime, then you will need to speak to a lawyer.
Yes, you can both be charged with the offense. However, whether you will be convicted is another story. If your girlfriend admitted that the paraphernalia is hers, you have a strong argument. I also agree with the other posters that you need a lawyer to look closely at the search warrant to see if there is any defect.
Theft by bailee is a person who holds another person's money and steals it. If you are an authorized user on another persons account but it is not your money and you use it for yourself, it would be theft by bailee. However, as a bailee you are usually authorized to use the money. It really depends on what you use it for.
If you are under 21 with misdemeanors, expungement is available. Not so for felonies. You can request a pardon from the governor although it is difficult. Contact the Governor's Office for the packet of information.
You are right to be concerned. A public forum such as this is not the place to discuss these matters. You need to speak to a lawyer at his/her office about these issues of drug possession and domestic abuse at once. These are very serious questions that need to be discussed and answered in a private attorney-client setting.
You definitely should schedule an appointment to meet with a lawyer. Bring the citations with you to that appointment. The tickets for paraphernalia are interesting since you didn't mention that you had anything like a pipe or other items. Good luck.
I would suggest that you call a lawyer in your area of Wisconsin. We can probably do something for you on the case if it's in SE Wisconsin. You're right. The fine isn't the issue, it's the municipal conviction for possession that you want to avoid.
The underage drinking tickets should not affect your auto insurance if the offenses occurred at house parties.
Different prosecutors handle these matters in different ways in Wisconsin. Most likely you will still be able to take advantage of the offer out of Winnebago County. However, Dane County may not make a similiar offer if they run a record check on her and find the other pending ticket. I would suggest speaking to a lawyer in Madison and get a price from him/her to handle this for your...