The in-flight entertainment (IFE) systems receive navigation data from the flight deck computers so they can display the moving maps and other stuff on the entertainment displays, for those passengers who want to know "where am I", "are we there yet", "is it time to reset my watch because we've crossed a time zone and I'm trying to adjust my body clock".
I would be shocked to learn that Boeing allowed the IFE to put ANY kind of data into the flight deck computers. I'd actually expect Boeing to use a one-way interface, one that transmits but does not receive: think RS-232 with one of the pins removed. I'd be almost as shocked to learn that Airbus did something like that. However, Airbus's comment about "firewalls" does not exactly inspire me to confidence in their airplanes.
That is the concerning part.
Are the systems accessible in the cabin physically and electrically isolated from all other systems from the plane? I don't think so. I think they are connected. And I think they are more connected that the companies prefer to admit.
First, are the systems physically connected? My money is on 'yes', because of the very reasons you listed. The IFEs are able to get data from SOMEWHERE, the question is where that is coming from. In computer hardware it is extremely rare to make a unidirectional connection. If nothing else you want to acknowledge receipt. They get data about the flight, they have connections for the phones for those who pay for it, they have connections for the expensive wifi connections. Do the companies really provide two duplicate sets of radios, one for the passenger data, a second duplicate set for operations data? Seems the opposite of every business I've worked with that wants to save cost.
Assuming they are connected, how are they connected? Since companies want commodity and standard equipment, I would not be shocked to see Ethernet. And if it was Ethernet, the comment that the seat boxes use a "modified Ethernet cable" is not too surprising, since the RJ45-style jacks are easily damaged. There are many more standardized sockets and jacks available, including plain old pin and head units.
That is the question whose answer I don't trust: considering how IFE systems get data about the flight, and how they like share external communication systems, it seems almost certain the systems are attached, even if it is "behind firewalls". If data can flow somehow, there is a way to communicate.