Do we ever find out how she was attacked? I'm not aware of Umbridge having any flying pets that might've done the job...

We get three moments of strong emotion. If we assume they're all related to the Ministry affair - we have no information about anything else, afaik - then we'd get something like this:

1) The night before leaving for Hogwarts; Voldemort is furious, and the next day Sturgis Podmore doesn't show up for duty. We learn afterwards that he's been arrested at the time Voldemort was furious. So this one seems obvious: he'd had him Imperiused (likely at the previous time when Podmore had failed to show up for duty) and now was furious that his new pawn got caught and arrested.

2) The happiness at the time of Harry's detention: The trial of Podmore is the only thing that makes sense. Voldemort must have feared that the truth would come out, that they'd realize he had been Imperiused, and perhaps even on whose orders. For a moment there the whole thing hangs in the balance, if Podmore talks his whole attempt to maintain secrecy will fail and he'll get Aurors on his tail. But Podmore goes to Azkaban without saying a word, and so Voldemort is happy at the narrow escape.

3) The anger later on: No particular clues for this one, except perhaps in the next chapter, I'll keep an eye out. Harry is most likely right for a change on this one: general anger that his henchmen's attempts, either to get in directly or to Imperius someone new to get in, are failing.

Yeah, noticed that too.