Good questions!
1.They will both need separate I-134’s, but they usually accept a Xerox copy of the original for a K-2 if they will interview together. (Check with the consulate’s Immigrant Visa Branch to confirm or you can just notarize the Xerox copy). Xerox copies of original supporting documentation should be good for the K-2 applicant.
2. Although I believe item #10 was technically meant for previous visa beneficiaries, I would go ahead and list them both here just to be safe. For relationship, use “fiancée†and “child of fiancéeâ€.
3. Duplicate copies means original plus a photocopy. All copies should include both sides of any pages with anything on the back.
4. The notarized affidavit is technically valid for a year, but some consular officers may prefer a more current version. I would say within 3-6 months should be O.K.  However, your supporting documentation should be as current as possible. Employer letters and payroll stubs should be dated within about a month of the interview to be safe (use FedEx?). If your income from employment is well above the minimum (100% of poverty level), then they may not even need to see a bank letter or evidence of personal assets, but to be safe you can submit the bank letter anyway. If it becomes a big hassle getting a signed bank letter, submit several recent bank statements instead.
I believe that the I-134 is no longer considered legally binding, but they use it primarily as a tool to judge your ability to support your beneficiaries financially.
They ask for photocopies of your most recent complete tax return, including W-2’s and all attachments. I would have the last 3 years tax returns available at the interview, just in case they see something that needs clarification. Submit most recent but have others available to cover you. Note: You will need the last 3 years tax returns anyway for the AOS I-864 later down the road.
Make sure you have the affidavit notarized by a U.S. notary!
Since you submitted a month ago before the new form came out, I was curious if you received your RFE yet, asking for info on past criminal convictions and Marriage Broker info. Did you meet through a “Marriage Broker†agency?
Ray