I wrote an article about this yesterday, nowhere as detailed as yours but I did have some of the same doubts you seem to have. mainly my thoughts are:
Why would Russia kill someone in the UK when years ago they could have done it while he was in prison in Russia for being a spy?

Why would Russia use a nerve agent which supposedly only they can produce, I personally would have used a weapon made elsewhere?

If the nerve agent is so deadly, how come none of the victims are dead?

Of course the attack has to be investigated and if it turns out it was Russia after all, use any measure available to punish this action, but with real evidence.