As written, I would use for. However, I wouldn't normally write the sentences that way. I would say: "Disaster was looming for the city. (Or "over the city.")
In the other example, I would say it thus: She was crushed by the thought of her daughter dying. Or: She was crushed by the thought of her daughter being about to die. Or, better:
I suppose it's possible; but in these particular sentences, the natural way would be to use impending as the adjective (1st sentence), and looming as the verb (2nd sentence).