The sound engine libraries you listed here are indeed meant for a game to link with and cannot be used for plug-ins. Accessing sound engine services from a plug-in is possible, but it is limited to what is offered to the plug-in context: AK::IAkGlobalPluginContext.
Some details on the infrastructure of plug-ins to understand the error: each plug-in library exports a linked-list of plug-ins it contains through the global symbol g_pAKPluginList. A corresponding linked-list in the sound engine SDK libraries contains the aggregated linked-list from all plug-ins, which is built at static initialization through the AK::PluginRegistration instances generated by the AK_STATIC_LINK_PLUGIN macro located in the factory file of the plug-in, or loaded as needed when using dynamic libraries (in which case the game does not include the factory file). Having both your plug-in and the sound engine SDK define g_pAKPluginList is why you get the "LNK 2005 g_pAKPluginList already defined" error.
Now, instead of querying for a specific state group, you should rely on the parameter abstraction so that any state group can be used and so that the mapping of values is user-defined.
To add an RTPC-enabled parameter to your plug-in that supports states, add the SupportRTPCType="Additive" attribute to a property in your plug-in XML definition; details can be found on the documentation page Plug-in Property and Custom Property XML Description. Refer to the AkDelay sample effect plug-in found in "SDK/samples/Plugins/AkDelay" for an example of such plug-in that supports states.