-What does "yast2 channel-upgrade-oes" do?
channel-upgrade-oes will upgrade the OES services, in your case from oes2015 to oes2015sp1. (during down server media upgrade, services will be upgraded automatically after entering the password)
-What happens if a customer fails to run this?
rpm's will be upgraded but not the services
-What are the consequences of not running "yast2 channel-upgrade-oes"?
since rpm's are upgraded not the services, system might be in inconsistent state
-Does it still need to be run if the server is restarted??
yes, until yast2 channel-upgrade-oes is run, upgrade is not complete.
-Can "yast2 channel-upgrade-oes" be run multiple times??
Yes, but its of no use, once yast2 channel-upgrade-oes is successful, running again will not have any effect
-If the customer fails to run the script at upgrade time, is it safe to run the script later??
no, yast2 channel-upgrade-oes should be run immediately after the rpm upgrade. As time passes, there is opportunity for configuration changes that the script may not understand in the ideal upgrade steps. If certain that no changes have occurred, run the script to complete the upgrade. Alternative method to address this issue would be to do a down server upgrade.