In the field configuration set Editable to No. This should prevent users without the manageentries capability from updating the field content.
For notifications you may want to consider the following workflow:
(0) Draft - Initial state.
(1) Awaiting review - Ready for review.
(2) Approved- Reviewed and approved.
Participants submit entries either as Draft or as Awaiting review. In Draft state participants can continue working on the entry and change state to Awaiting review when ready.
Instructor can change entry state from Awaiting review to Approved or back to Draft if revisions are required.
With this workflow, to count Approved entries in the activity grade calculation you should use the following formula
This formula retrieves only '2' values from the field, that is, only from Approved entries, and so the sum divided by 2 gives the count.