node_view not picking up view mode, always printing full
I think you have a misplace parenthesis.  You have

$form[...] = drupal_render(node_view($eventInfo['event']), 'teaser');


The view mode should be a parameter to node_view() and not drupal_render():

$form[...] = drupal_render(node_view($eventInfo['event'], 'teaser'));


As a side note, I don't think you need to explicity render the node and stuff it into a #markup.  node_view() returns a render array with a #theme already defined, so you should be able to do:

$form['commerce_fieldgroup_pane__group_event_information']['field_event_information'][LANGUAGE_NONE][0] = node_view($eventInfo['event'], 'teaser');