Magento – Override abstract class in Magento 2

Technically, you are correct, you can override the class in the local code pool, but you should not. When you override a class, you own it, meaning you must now maintain it. This includes upgrades, etc. It is very bad practice.

Second, since an abstract class is never instantiated through the abstract factory pattern, (Mage::getModel(), Mage::getSingleton(), etc) it will not work to rewrite it through configuration.

Third, when you rewrite such a low level class, you should seriously reconsider your approach. What are you trying to accomplish? Are you trying to rewrite the collect() method for over 24 classes? I ask because this is what Mage_Sales_Model_Quote_Address_Total_Abstract is inherited by. Perhaps you should rewrite a method call from a more specific class. Maybe check into an observer that will solve your issue.

To answer your questions listed above:

1. Extending inherits the rewritten class and you only have to change the functionality of a particular method, (not the entire class).
2. The xml above is rewriting a class. Putting items in the local code pool overrides them.
3. Autoloading is a bit of a big topic, but simplistically:
   1. A class attempts to instantiate.
   2. The autoloader maps the class name to a file name, (example: Mage_Catalog_Model_Product maps to Mage/Catalog/Model/Product.php.
   3. Magento attempts to find this mapping in one of the three code pools (local, community, core) and in the lib directory.

A google search of autoloading in Magento will give more in depth process of autoloading.

First of all the constructor is private api of class. Constructor function have special meaning and do not require have to have same list/order of arguments as in parent class.

Is it possible to inject a dependency into a Magento 2 CRUD model?

Yes of course.

Is this safe?

Yes, but Magento Object Manager assume that all optional parameters is placed at end of list and required parameters after optional will not resolved.

$resource, $resourceCollection arguments is legacy but still widely used in Model classes. Most of model use code like this to initialize resource and collection class.

protected function _construct() { 
    $this->_init('Magento\AdminNotification\Model\Resource Model\Inbox'); 
}

This is why this parameters is optional. But, for example, in unit test we pass resource or collection mock in constructor to allow replace realization.