Hi John,
The information you're looking for can be obtained by looking at the TextChange property on the EditorSnapshotChangedEventArgs that you are already handling (which is an instance of ITextChange). The ITextChange.Operations property (which is a collection ITextChangeOperation instances) provides access to all of the changes that were made.
The ITextChange.Type property can help you determine the type of change (e.g. Delete).
You can use properties like ITextChangeOperation.StartPosition and ITextChangeOperation.LinesDeleted to know which line numbers were deleted. For example, if StartPosition is line 20 and 2 lines were deleted, you know that lines 20 and 21 were deleted.
That should get you started in general. If you run into challenges with specific scenarios, please reach back out.