We found out, that a role corrector cannot edit grades in singleview, because he hasn't the capability moodle:grade/manage (grade/report/singleview/lib.php)
However he has the capabilities moodle/grade:edit and moodle/singleview:view.

We think, that the requested moodle:grade/manage should be replaced by the moodle/grade:edit capability in the function

public function process_data($data) {

if (has_capability('moodle/grade:manage', $this->context))

{ return $this->screen->process($data); }

} 

in line 54.

To edit grades it is not necessary to be able to manage other settings related to grading.