is it possible to overcook soup?

of course...you don't want mushy noodles in any noodle soup. You don't your veggies to disappear into a puree in any vegetable soup.

Yes,if it reduces too much it may become too salty.

Yes, the 'bits' can become too soft. and the soup may get too thick and concentrated, more like a puree than a soup.

Yeah, if the vegetables or noodles or meat are mush or deteriorated.

yep! it all turns to mush! No visible bits of anything!

Absolutely my friend. Its cost me big time a couple of times. I fell asleep with the pot on. You know the result of that.

Boil the liquid off and the other ingredients will burn....