I think the problem with "Are there any of the 10 core worlds without a shard?" is that there is is no time frame specified, which probably factors into Brandon's answer, muddying the waters even more. Sel comes to mind. And it's possible that a core world could be one that the nomadic shard visited, but would you say it did/did not have a shard? It might be worth asking if the 10 core worlds either had or will have at least one shard.