Assuming the A380 is like other Airbii, you can shut the engine down via the Master Switch or Engine Fire pb. I agree, if these 2 are routed via the same loom, that is not sensible... and some sort of separate paths should be required. However, if the routes are separate, and were taken out by bad luck, so be it.
I think there ought to be a better way. Sure, you could feed the signals down two looms but then you go back to the old INS argument -- which signal do you believe? One data bus says "stay as you were" and the other says "shut down" so you add a third that might say "dunno". Someone on this thread floated the idea of a radio control system that could be enabled in an event like this -- would still require that electrical power was present at the control servos and solenoids, of course, but might be a relatively inexpensive and reliable 'emergency network' that could be used not just for engines but other commands and controls? There are other ways but, as you say, failures by their very nature tend to be unpredictable when sharp metal starts to fly around.
I'd rather have engnes running when I didn't want them to, than numerous stopping methods / boxes that mean I have engines not running when I do want them.
Hmmmm.....I lost a throttle cable in a piston-engined Jodel, thirty or more years ago and the engine opened right up on the spring -- which it was designed to do. One of those things you read about and theorise about but when it happens you feel rather glum. But in a small piston-engined aircraft you have mixture and mags, so you can go idle cut-off, mags off, then and mags on and rich to blip as they did in the old days. Not a big deal in a Jodel but I don't suppose it would be so easy in an Airbus? And you can't blip a Trent. With powerful power plants and aircraft that don't glide well an engine that cannot be controlled must be a major design defect that needs to be corrected. Why did the 195x Jodel have a greater redundancy than an Airbus?