"making such a pig's ear of a straightforward assassination.
your knowledge and understanding of how such matters work in these instances is severely lacking.
If a straightforward assasination was wanted it would have been done with a gun and a silencer...it was not wanted.
The russians are playing a game as they always do...they know everybody knows that they did it, but no matter what will never publicly admit to it.
This is a warning to any and all enemies of the state, we know where you are and we can get you at our leisure and with a method that suits us.
If you know anything about this family of poison agents (pun intended) poison you will know its unreliable in how it works, eg : how its applied in relation to its effectiveness, time it takes etc etc.
This may well have been botched (unlikely)or it may have been an experiment in its use , either way the message will have got through.
If they wanted him dead he would be