First, you need to see a criminal lawyer. The lawyer will find out if you have been charged with dui. Was anyone else injured in the accident? It is possible you were or will be charged with something more serious. Once you know you have been charged with a crime and what the crime is, see an immigration lawyer.
If you marry, the US does not care if your fiancé remains a citizen of the Netherlands.He or she can get a US green card. When or if your fiancé wants to become a naturalized citizen of the US, whether the fiancé loses their citizenship in the Netherlands depends on whether the Netherlands requires it.
This sounds like a real problem. You need to prove that your marriage was legitimate. If you have a conditional green card, you will need to apply before the 2 years runs to remove the conditions. If within 6 months of the marriage you left to live with another woman, it doesn't bode well to prove you had a legitimate marriage. I really recomment that you consult with an attorney as soon as possible.
You really should retain counsel to assist with this process to make certain that it is all done properly. When you consider what is at stake in the event there is a bad result, you will agree that it is money well spent. A good immigration lawyer is likely less expensive than you think. I doubt that any lawyer on this website will give you a complete list of all documents that would helpful to support your application since each case differs based on the particular circumstance. Plus, a good...
You may not have anything to worry about since it was a misdemeanor. But, how would it help to worry? Worrying about tomorrow saps the energy out of today. At this point there is nothing to do except wait. If you get an adverse result, speak to an attorney immediately.