You make some excellent points in your post. I've been following this story but I couldn't write anything as I cannot make up my mind who's telling the truth. My first assumption is that it's a false flag operation and the information that Skripal had some connection with Steele makes it more likely. Also, the ability to quickly identify the poison - raises indeed a lot of questions... if Western powers have the formula and samples, I guess they could all make it (No doubts they have some stockpiles...regardless of international treaties)
On the other hand, there's the tantalizing possibility Russia might have done it, which would mean Putin has grown some incredibly big balls!