Given how much damage an unrestrained high-end Exalted can do, how the heck did Donovan Hock manage to kill Keiji? And even if he did manage to do so when he was not near her, why did it take so long for her to get back at him?
It was mentioned somewhere that Kasumi was drunk and/or away at that moment, and thus wasn't able to stop Hock. Not sure for the second question. Maybe she was setting up the perfect revenge? (Waiting for a big party to humiliate him, getting and training a bear and honey badger, ect.)
Yes, Kasumi specifically mentions that she made a mistake that left Keiji vulnerable. Exalts are immensely, awesomely powerful, but they're still human. They make human mistakes.
The specific flashback when Keiji dies mentions that Kasumi actually had a weapon leveled at Hock and wanted to pull the trigger, but she was also seeing at the same time that Kaiji was fated to die there and was apparently unable to interfere in that fate. It's that which greatly affects her breakdown. She's also not that well-trained in her Exaltation, going by the conversation between Logos and Pria.
Sidereals are also particularly limited in combat capabilities, particularly when compared to other Exalts. They can't make custom charms without the direct influence of the Maidens (which is probably not something Kasumi can get), and their Perfect Defenses have incredibly wide loopholes. Avoidance Kata can't be used after two combat rounds, Serenity in Blood fails if the Sidereal fights their own astrology, and Duck Fate is both incredibly expensive and randomly fails. Martial Arts, especially Sidereal Martial Arts, can fight that effect, but they require a lot of infrastructure to learn or experience to build from the ground up. With a lot of planning, Siddies are incredibly powerful and practically invincible, but they don't have Solar-like abilities to just punch people into space.
How did Chekov go from being a Giant Panda...to a grizzly bear?
Where was it ever said that Chekov was a panda? Chekov is a South African Honey Badger, and is a completely different creature from the grizzly in the box. Kasumi even included a note that explicitly said "This is not Chekov."
Maybe it was just me reading into things, do apologize