Hi,
I suppose you'll want to use the same trigger "After an order has been updated" in your mass action.
In your action's code, you could look at the values in $_REQUEST.
There, you'll find the controller with the key "ctrl", the task with the key "task", etc
I would recommend you start with this PHP code in your mass action:
<?php var_dump($_REQUEST);
That way, you'll get a list of all the data passed by the browser to the server when an order is being modified and you can then adapt your code to get the information you need for your checks etc.
For example, when a product is deleted from an order, the ctrl key will have the value "order", the task key will have the value saveproduct. Then, you'll have
$_REQUEST['data']['order']['product']['order_product_id']
with the order_product_id of the product being deleted and you also need to check that
$_REQUEST['data']['order']['product']['order_product_quantity']
is set to 0. If it's not 0, then the quantity of the product might be changing (or not).