How was Usagi reincarnated

after the fall of Silver Millennium if she didn't originally have a Sailor Crystal?

As per established lore, all beings have a Star Seed, but only some have a Sailor Crystal making them capable of reincarnating.

This being the important distinction, as per the manga, that put the fates of our beloved Mauian trio in limbo.

However, that posed a question relative to Usagi.

One could argue that Mamoru retroactively had a Sailor Crystal, the Golden Crystal, but Usagi was never intended to be a senshi, so...

The Shitennou would have the equivalent of the Amazon Stones, so that resolves that issue.