There is a provable trail
"having been poisoned with the isotope polonium-210 by sinister forces whose identity is easy to guess but hard to prove"
Nope - not in this case. Very easy to prove. However, as the Russians will not allow extradition or questioning of their nationals then it hits a brick wall. The evidence trail, cause by the polonium-210 itself is compelling and points directly to the immediate culprits, although whether such individuals could be persuaded to provide evidence to convict their masters would be another issue. In fact the cheif suspect is a member of the Duma and beyond the reach of law.
As it is, with the Russian state on their side it's academic.