Am I the only one who read that quote and was completely unsurprised? The only thing that was unexpected was that Feruchemy "changed" the focus, so now Allomancy grabs a Feruchemical ability instead. I hadn't heard of Compounding described in exactly that way, but it makes perfect sense.
All Brandon is saying in that quote is about how focuses work: a specific thing which filters the power in a specific way. And yes, it bothers me that we don't know the focus for Surgebinding. That's why this five page topic dedicated to blabbing about focuses exists. Um, yeah.
First thing, I don't quite agree with your analysis of how Shards work in that initial paragraph. Needlessly complex. I prefer to think of it as the Shards are the power of creation--they aren't hooked up to an external power, which is what your statement sort of implies.
And yes. Focuses are the method which humans get to use that power. From the Brandonology (Hero of Ages Spoiler thread):
How do you access it? Through the metals. From the Hero of Ages Annotations (Chapter 38, if you're wondering):
So, yeah. None of this stuff is exactly groundbreaking.
Regarding the Commands, I always thought that that focus was a Cognitive focus, rather than a Physical one, like metals or the shapes of Aons, and that's perfectly okay. It's more the intent behind the Commands that make them work, as you say.
One more thing: focuses don't have to explicitly link to the power of creation. Look at Feruchemy, which draws from your body, rather than from the external power of creation. It's an important subtlety to keep in mind when trying to define what a focus is. But all a focus really is is a specific gateway to a specific interaction. At least, this was what Mad Scientist and I determined in the Focuses on Roshar thread.
View that quote from the annotations I posted there.
So, yeah, I don't mean to be a downer, but we seriously knew all this stuff before. Indeed, focuses are extremely important to magic systems and the cosmere in general. I don't mean to denigrate that part of this discussion. Identifying a focus is absolutely essential. It's just we knew that already, years ago.
Some more astute questions would be, why is a Shard's magic tied to a particular focus? Why are Ruin and Preservation blind to their focus? What are the focuses for Surgebinding? What is Endowment blind to? And so on.