You say this, but then I used to read r/justrolledintotheshop. People outright fail at substituting engine oil for engine oil from time to time. The kind of person that would try applesauce probably wouldn't be doing more self harm than otherwise.
(Fun fact: putting gasoline in a diesel engine is way worse than putting diesel in a gas engine. In the latter case, it won't run until you replace the fuel with the correct stuff. In the former case, it pretty quickly destroys the engine.)
Yeah, the fuel injection on modem diesels requires very high pressure and the pumps that form that pressure rewire very tight tolerances/clearances and rely on the fuel as lubricant. Without it, they grind themselves up, ruining both themselves and the injectors downstream with a shower of metal particles.
The entire fuel system has to be replaced.
I think it depends, I've known of someone doing it in an old wreck years ago, and after refilling with the correct fuel it ran as well as it did before (which is to say it smoked and sounded like a bag of spanners). No idea how much it may have shortened what life it had left, but it wasn't instant death.
Thanks for letting me know, 8 didn't actually know that. I don't think I'll ever own a Diesel vehicle but I'll keep it in mind that it's not so simple as emptying the tank lol