Even if Borg memories can be stored forever, they won't be stored forever. Using a virus or destroying the equivalent of their version of hard drives would be effective in destroying memories. If the Borg got into a war with a formidable enemy that destroyed a bunch of Borg ships or some Unimatrix complexes, then Borg memories from before that war would become hazy.
Not sure that's how it works to be honest. Based on Seven's dialogue, it would appear you would have to wipe out the entire Collective to completely erase everything, not just parts of it.
Personally, I chalk up her "fragmentary" comment to lazy writing and not wanting to spend the time to come up with a detailed explanation of Borg history.
Yes, and no. I don't think a single drone can hold the entire history of the Collective. There is probably redundancy, but kill enough drones and you'll start to corrupt the data storage.
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My character Tsin'xing