Nonclustered Index still doing index scan
Try to create this index:

CREATE NONCLUSTERED INDEX idx_dmcasarms_GradeSubmissionList_RecordIDEdpCode
ON [dbo].GradeSubmissionList(EdpCode, DateSubmitted)
INCLUDE (FinalApprovedRejectedBy, FinalAction, FinalActionDate, FinalActionReason );


Index Seek  will be used only if conditions from WHERE or ON clauses are the first in the list of columns of the Index. Also, this fields must be selective which means that the condition must filter just a little percent or rows from a table.

Also, you don't need to include column that is part of the clustered index into list of columns of a nonclustered index, because Nonclustered indexes always contain the clustered index columns if a clustered index is defined on the table.

Actually, this is very complex subject. To know more about using indexes in Sql Server you can read this article SQL Server Index Design Guide