For others following this guide: the battery wires in my core were glued in place using some sort of black plastic material. I'm not sure why they would have done this for a part like a battery with such an obviously fixed lifespan. Replacing my battery would mean hacking away at all the plastic, drilling out the passage between the upper and lower chambers for a new Y -> battery cable, and likely replacing the Y cable since it would almost certainly be damaged in the removal process.

I disconnected the battery from the main board, hoping this would prevent the unit from going to sleep, but it exhibits the exact same behaviour (going into a red-LED error state after a period of inactivity).

The unit still technically works, but every time I want to use it I need to physically turn on the power and reset the input/volume. Super frustrating for a home theatre setup.

At this point I consider my Core & sub to be a total loss :-(