I think we need:

$event->set('languageCode', $SESSION['survey'.$surveyid]['s_lang']);

$event->get('templatedir') : because else : We can not update templete : we only update CSS/JS call, but NOT call of all .pstpl

And i think we can add:

$event->get('format')
$event->get('allowprev')
$event->get('questionindex')
....

All getsurveyinfo except : Set not-anonymous to anonymous (because it can break Answer save (no token colum).
But maybe whole is OK: plugin dev must be carefull too.