[Ed B.]

Thank you very much. O.K, I got that
There is a function markAsOrder in the Model/Repo/Cart.php
which is used in Model/Cart.php

    /**
     * Mark cart as order
     *
     * @return void
     */
    public function markAsOrder()
    {
        parent::markAsOrder();

        if ($this instanceof \XLite\Model\Cart) {
            $this->assignOrderNumber();
        }

        $this->getRepository()->markAsOrder($this->getOrderId());
    }

which in turn is called in Model/Order.php

    public function processSucceed()
    {
        $this->markAsOrder();
....

So I presume I only would have to modify somewhere in the chain the value of is_order, I am done? And probably the simplest thing is to decorate Model/Order.php to have something like

    public function processSucceed()
    {
        if (!isFake()) {
         $this->markAsOrder();
          }
     ....

And maybe to be on the safe side, I can add couple of lines to set is_order=0 when I have a fake order?