Thanks for the responses!
Perhaps I should clarify in the first question... Brandon has mentioned things in the cognitive realm really only having 'shadows' in the other realms. Could things like this take up a shard? I'm assuming not, because although Kelsier seems to have done something to help contain Preservation after he died, he didn't fully take up the shard. I'm assuming because he couldn't. Anyway, thanks for the response. It makes sense that natural existence in all three realms would be required.
Also thanks for the clarification on the second part. I assumed that's what it was, but after hearing the cognitive realm referred to as Shadesmar so many times I started to wonder...