>base64'ed DES-encrypted primary int64

Block ciphers (such as DES) don't collide. If they did there would be no way to decrypt them because a block could be decrypted into multiple valid plaintexts.

Not sure about technical details for that but perhaps something like https://vitess.io/docs/reference/features/vitess-sequences/. Vitess itself is a project that grew out of YouTube's needs to scale and shard their MySQL DB.

Probably they just roll again. You can even implement that in a stored procedure.

Try again? Just a guess.