Multiple inheritance is actually a dangerous thing, as it can make an object's behaviour unpredictable. Having the same methods with the same signature defined in more parents makes things unpredictable, as it is difficult to know for the compiler to assign which behaviour to the child. In C++ it is recommended that you only use multiple inheritance when you are a really really really really really skilled C++ programmer.
Java also has only one parent object a child can inherit from. But they made a work around for it in the form of Interfaces "Objects without methods defined, only declared."
In GM that same problem arises when you use event_inherited. If more than one parent has a behavior for that event defined either through its parent or by itself. Then the GM compiler would not know which behavior is executed.
Weird that my problem does not happen with some one else. Maybe it is the how variables are declared.
EDIT: Okay thnx to Xfixium the problem is solved. I feel like a major idiot and n00b for making such a basic mistake. Forgetting to include event_inherited in the last child of the chain and not even seeing it when I go over it a million times.
*** Leaves to sit in the corner with a Dunce hat on. ***