I have been testing the new assignment attempts feature in Moodle 2.5.1 and found the following.
Require students click submit button: Yes
Require that students accept the submission statement: No
Attempts reopened: Manually
Maximum attempts: Unlimited
Students do not submit in groups and there are no other extra settings.
Student uploads an assignment.
Teacher (by mistake) locks the submission and also adds another attempt to the student. Student cannot modify the submission. Teacher cannot see "Allow submission changes" in the quick menu in the Edit column (only Grade and Grant extension appear if the status is reopened).
The only way for the teacher to unlock the submission is to select the submission in the grading table and set the unlock option at the bottom.
I think that the Allow submission changes should appear in the Edit menu.