Why you are creating new actor instances via Activator.Createinstance (EPiServer.Forms.Core.PostSubmissionActor.ActorsExecutingService)?
Please imagine the Actor is kind of "static" mission, executes after form posted, similar to
ActorsExecutingService is used for executing the Actors. It is the default implementation and you can replace its business if you don't like the its business workflow.
I don't think it is kind of generic purpose instance that you instantiate and use it solely. If you need that kind of freedom in specific project, using the Event mechanism is better.