Where is AVExtensionMgrRegisterNotification, or AVAppRegisterNotification?

That's good. We hear from a lot of people whose problems flow from trying to learn C++ for the first time at the same time as trying to write their first plug-in. As we know, a lot can go wrong...

Not overkill, but mandatory. You cannot load plug-ins into Reader unless you get a Reader Plug-in license. They just will not load unless signed with an Adobe-supplied key. Reader plug-in licenses are only granted in limited situations: they are not allowed to add functionality found only in Acrobat. Prices are variable: the most expensive are plug-ins that implement security. These "DRM plug-ins" typically cost $50,000 per year plus royalty payments ( Developing digital rights management (DRM) plug-ins | Adobe Developer Connection )

So even the simplest experiments will need Acrobat. Bear in mind you can subscribe. Notice cancellation terms: the cheaper subscriptions are for a year commitment.

You haven't said but I suspect you may be making the other Big Mistake so popular with plug-in beginners: you are trying to avoid writing in C or C++. No, you cannot. Furthermore, to avoid complex issues, use ONLY the version of Visual Studio documented in the SDK you are using.

I didn't answer because the services are not provided by a DLL. Your question suggests you are trying to just write new code using the APIs: do not do this. I said Acrobat not Reader because that is what you need to do. Samples can only be used with Acrobat. You will need to license Acrobat and use Acrobat initially, before finally applying for your license to write Reader plugins. 

The plugin API does not use DLL entry points. All the linkage takes place when Acrobat loads the plugin. You should start by compiling sample plugins and attempting loading them in Acrobat (NOT ACROBAT READER) until this works and things start to make sense. Only then start to adapt for your needs.