There is no law that prohibits a police officer from asking an arrested person to sign documents. In WA, the arrested person does not have to sign or agree to sign any document take make the arrest valid (if the arrest is otherwise valid). The failure of the arrested person to not sign documents does not make invalid any document (such as the notice of license suspension) the officer serves on the arrested person.
As always, before signing anything, the person should review the documents and understand them before signing any documents.
Sometimes police officers do ask arrested persons to write statements or sign statements prepared by the officers. An arrested person likely should invoke the right to remain silent and the right to see an attorney.
Blowing "below a .1" can still be above the statutory 0.08 threshold. Perhaps you meant 0.01.
Even a person with a level below the statutory threshold can still be found guilty of DUI if the prosecutor can prove that the person's ability was impaired.
You should review your specific facts with your attorney to find out your legal options.
There are three documents where there are four places for the accused's signature. How the documents were presented, when your rights were read, the specifics on how the "processing" of your arrest is critical in determining the validity of those documents. Did you get to speak with an Attorney? Were you given the documents to read, do you wear eyeglasses and if so were they with you, were you handcuffed, if so, then how were you able to read the documents, were the documents read to you? There are many questions to ask. You have good instincts in picking up on the specifics on the documents to sign.
There is no obligation for an arrested person to sign anything at all. In fact, signing nothing is a good course of action. Some documents simply prove that you were given certain warnings, or certain documents, while other signature lines in a DUI processing, however, is an indication that you understand your legal rights and freely give them up. Sometimes these these documents are all presented to you in quick succession and at the end of the processing procedure with an "x" marking the spot.
Consequently, a person should always read everything they are being asked to sign, and if in doubt, do not sign the document. The best course of action, regardless of whether you actually understand the documents, is to invoke the right to speak with an attorney at the time of DUI processing. While an attorney of your choice may not be available, a public defender should be available for a consult by telephone, regardless of the hour, and you can ask questions about signing or not signing. If you are having second thoughts about what you signed, be sure to tell your attorney the specific about how the document was presented prior to signing as well as the circumstances. It is best to consult with an attorney who focuses their practice on DUI defense.
Since you are in the Seattle area, it is likely that charges will not be filed right away, but do not wait, the Department of Licensing procedure requires action within 20 days of the ARREST so consult an attorney quickly and write down the sequence of events now when they are fresh in your mind.
The officer cannot force you to make statements, written or verbal that admit guilt or are in response to questions he makes once he has arrested you. I have never seen a statement prepared by law enforcement that is given to a defendant that is an admission of guilt. Typically, the signatures they ask for indicate that you have been read your constitutional rights, that you agree to waive those rights and talk to them, and that you have had the implied consent warnings.