We need to reschedule our biometric exam because it occurs on a day that we'll be out of the country. The I-797 NOA states that if we need to reschedule, we should send a copy of our request to the service center that we filed our Petition to Remove Conditions on Residence. That service center is Nebraska, but our case was forwarded to the CA one. So... should I send the request to NE or CA?

You may be better off scheduling an Info-Pass appointment to your local field office, and either rescheduling it there, or perhaps get the biometrics done at the info-pass appointment.

http://infopass.uscis.gov/index.php

US YOU http://www.visajourney.com/timeline/profile.php?id=9445

Too few have timelines for lifting of conditions.