The reason for extending the acf.ajax property is in order to be able to access other properties of acf.ajax more easily. I’m sure they can be accessed directly, but I it’s just easier to do it the other way by following the code in ACF as an example.
I don’t know when you were trying to add your actions, but ajax actions fire immediately after the admin_init hook, so as long as you add them before this hook fires you should be good.
