I would argue the Returned are a special case, and not directly related to the case at hand of Snapping/Shaod. Brandon said in that recent Warbreaker Q&A that Returned are known as "Splinters" in shard cosmology. This sets them apart from the other magics we have seen. Including them in this discussion would not be beneficial.
Awakening, however, may have a Snapping/Shaod mechanism. It's a very new art, so who knows? Just a thought. Now that I think about it, the only conditions for Awakening is 1. Having Breath, and 2. Knowing the Command. Since a child can say a command to give Breath to Returned, this suggests they have the ability to Awaken. The verbal commands for Awakening are actually its focus, as Brandon told me personally (metal is the focus for Metallic Arts, Aons for AonDor). If they have access to a single command, it can be possible that they can Awaken entirely. In which case, there is no evidence for a corresponding Awakening-Snap. (Awakening of Awakeners?
)
On the other hand, there are exceptions to Snapping with lerasium. Saying the Command to transfer Breath may exist outside of classical Awakening, and fall under a realm of a special quality to Endowment. Certainly, I don't remember if the transfer of Breath requires color. Awakening (for some inexplicable reason) requires color. If Breath transfers do not require color, it is not Awakening.
If that is not Awakening, then classical Awakeners may "Snap" through some method that would not be known due to the lack of study of Awakening, relatively speaking. Whatever that Snap is, it is less dramatic than Allomantic Snapping or the Shaod. The fact that it is less dramatic provides evidence that Awakeners may not Snap, and may exist under some different mechanism of Endowment, perhaps.
Then again, going back to Scadrial magics, there is no documentation about Feruchemists Snapping. This does not mean they don't Snap (in fact, one could argue that with the Lord Ruler's breeding policy castrating males, that pain could cause Terrismen to Snap). And here's another concern: for Hemalurgy, in order to steal an Allomantic power (or any power), could I spike it from an Allomancer who
hadn't Snapped? I don't think anyone has asked that question before.
Phew. I'm ranting. Sorry
The Shaod may not be necessarily random and probably requires specific genetic and spiritual codes by whatever wacky Shards exist on that planet, but Snapping requires an input of energy on some level (namely, pain) to reveal these abilities. The people the Shaod affect may not be random, but we do not have any data to conclude
why someone becomes an Elantrian on a particular day. There's clearly a methodology to the Shaod that we do not yet understand.