I know this thread is getting a bit old, but you can add some javascript (jquery in this case) to your theme to make sure the grading panel is the first panel to show up (as opposed to the split screen). It's just a matter of changing which panel is 'active' to begin with:
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if ($('body#page-mod-assign-grader.pagelayout-embedded')) {
$('button.collapse-none').removeClass('active');
$('button.collapse-review-panel').addClass('active');
$('[data-region=review-panel]').addClass('collapsed');
}
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