Within the Arcadia framework can you have an actor which only interacts with another actor and not the system?
Of course. And it is recommended if it is relevant for you and the design of the System of Interest (SoI).
For instance, for some systems the final user is not directly in contact with the system, as there may be a system between them handling the interface.
An example: a Cruise Control System may not be directly interfaced with a driver, as it may receive signals from the vehicle infrastructure. Still the driver is a key actor as she/he defines the desired behavior of the cruise control
Yes, but remember that the objective of System Analysis in Arcadia is to define the expected behavior of the System and its external interfaces.
No to model the external Actors!