Welcome here!
Have a look at this great photo by Ibsen:
The screw in the center (just above the top of the alternator cover in the picture) is fixed by two clips from the steel rail so it should not go inside that far as in your case.
The copper washer in your picture indicates that the nut was probably there some time ago. The surface does not indicate a broken bolt, therefore I think it still has its length. Basically it should be fine if you are able to pull back the screw far enough to fix the nut again.
But no idea how, maybe a strong magnet is an option.
Note that this bolt fixes a moving part inside. If this went out of position, the bolt might not like to come out any more.
Anyway worth a try.
One could even try to get a welding electrode "cemented" to the stud as a last try before you open the engine.
Possibly it helps to loosen the upper bolt a bit but of course things might become worse by that, too.
There is only one thing that I can say for sure: I really do not envy you!
