The return should be a dictionary which keys are the ids of the processed object and the values are the returned values.  I see that you are setting it as type many2many.  x2many has a special way to specify the entries to add.  For many2many the returned values need to be in the form of [(6, 0, [XXXX])]  where [XXXX] is the list of IDs that you want to link.

Also you are mixing v8 sytax with v7 syntax.  I'm not quite sure if it is OK.

So your method should look something like:

def _orders(self, cr, uid, ids, fields, arg, context=None):

    res = {}

    statement = self.browse(cr, uid, context.get('active_id', False), context=context)

    _order_pool = self.pool.get('sale.order')

    if statement and statement.partner_id:

         for _obj in self.browse(cr, uid, ids, context=context):

            _orders = _order_pool.search(cr, uid, [('partner_id', '=', statement.partner_id.id), ], context=context)

            res[_obj.id] = [(6, 0, _orders)]

    return res

To pre-populate the orders, you need to create the "account.bank.statement.review.wizard", passing the appropriate context, before displaying the view while passing the domain [('id', '=', created_wizard_id)].