What can I do if I have an incomplete K1 requirement

You must follow whatever instructions you received on the NOA immediately.

Generally, a Request for Evidence (RFE) will be issued to you. At this point you could send the supporting documentation in and ask that it be forwarded to the assigned officer using the receipt number that was issued to you. However, in most cases CIS will sent the RFE and request what you have not provided and then provide a cover sheet and deadline for you to return the information to them. I am always concerned that we when I send something to CIS that it will never get to where it is supposed to go when I do not have a cover sheet to place on top of the correspondence to see that it is correctly routed. You should always send as much as you have when you file because CIS can deny the application if required paperwork is not provided. However in a case like this, I would assume that CIS will do the right thing and will issue an RFE and ask for the supporting paperwork. Good luck.

Yes, if you do not submit the required documents in a timely manner, the application will be denied or deemed abandoned.

Hopefully when the petition is adjudicated, the CIS officers will notice these deficiencies and give you a chance to submit them in a Request for Evidence. However, sometimes, if it is the basic evidence that is missing (proof you are a US citizen and free to marry), an officer may deny for failure to qualify. Either is possible.

If any required documentation is missing, the officer adjudicating the case will most likely issue a Request for Evidence (RFE) and grant you additional time to respond.