This is because the content from the left column exceeds the column's width and as such is placed below the right column. It's really a matter of how your current Joomla template styles/supports K2.
Replying to your other 2 questions...
1. Yes. The output is a <ul> either way with 2 <span> elements for label and value. If you set a fixed width for the label, you can balance the layout in 2 columns.
2. Yes. Just move the extra fields block right below your "fulltext". Your template probably has overrides for K2 already under /templates/YOUR_TEMPLATE/html/com_k2/.
See the source template (item.php) here:
github.com/getk2/k2/blob/master/components/com_k2/templates/default/item.php#L244 
