Steve, it looks to me like your instructor is using the wrong tool.
In 2.4, you should be using the assignment setting "Offline grading worksheet" set to yes.
Instructor goes to the Assignment. "Grading action" choose "Download grading sheet." That downloads a csv. Leave the students who have not submitted blank, put grades in those that have. Upload via the Assignment "Grading action" = "Upload grading sheet."
Only the grades that were added are affected. No override.