You can use a role override to do this. You do this in the pull down menu for an activity, choose Assign roles, click on a role name such as Non-editing Teacher and add the user: they will not be able to edit the settings for the activity but will be able to grade it etc. because now they have the role of Non-editing teacher as far as that one activity is concerned.
The ability to edit the settings is all or nothing: if you can use Edit settings on an activity, you can change any of them. So you have to take that away.
You might need to create a new role for this, but using the Non-editing teacher is a good place to start. Manage activities is a course wide capability but each activity has its own capability for adding it to a course, eg. Add a new assignment mod/assign:addinstance, which are all Not Set for the Non-editing teacher role.
The ability to edit the settings is all or nothing: if you can use Edit settings on an activity, you can change any of them. So you have to take that away.
You might need to create a new role for this, but using the Non-editing teacher is a good place to start. Manage activities is a course wide capability but each activity has its own capability for adding it to a course, eg. Add a new assignment mod/assign:addinstance, which are all Not Set for the Non-editing teacher role.