You do not need to use any of this for just the USB part, of course. I'm just including these as extra features because I had some pins left to handle them, and I don't like unused pins much.
I dont understand what I could damage? I'm triggering interrupts with an open collector driver, just as the internal logic in the CPC. If the PlayCity doesn't handle that, how does it works? Are there schematics for it somewhere? As for the cursor signal, I'm only using it as an input, and I see no way this could damage anything. The only risk I see is triggering multiple interrupts at the same time, which is not really a problem and easily avoided if you program each device properly.
I know that the interrupts can conflict with the gate array, there are ways around that - either reset the GA counter often enough, or disable the INT and use NMI. I don't plan to use vectored interrupts. It is possible to check Albireo's status register, and if there are no flags set there, it means the interrupt came from the gate array or some other extension.
I'm out of dipswitches for hardware disabling of the cursor signal, but I can include a small solder link so it can be (relatively) easily disconnected.
I have started writing the developer documentation while I wait for the chips to be delivered. It is only an early draft now, but here you go:
Shinra Team