Hi,
1. I'm not able to reproduce the issue. The order_user_id is correctly changed when changing the user in an order on our end with the latest version.
2. Change the line:
echo JHTML::_('select.genericlist' ,$paymentvalues ,'new_payment_method' ,' class="inputbox" size="1"' , 'value', 'text', '', 'new_payment_method'.$row->order_id);
to:
echo JHTML::_('select.genericlist' ,$paymentvalues ,'new_payment_method' ,' class="inputbox" size="1"' , 'value', 'text', $row->order_payment_method.'_'.$row->order_payment_id, 'new_payment_method'.$row->order_id);
in the file "listing" of the view "order" for your frontend template and that should fix the problem.
3. This looks like a server error. Turn on the error reporting on your Joomla configuration and try again. Do you get any error message ? If so what message exactly ?