As far as I know, is he still a dual national, or do you know if he has renounced his RUSSIAN citizenship?
Furthermore if that were the case then the Russians would know this for sure, and would not be perusing this angle with such enthusiasm
Under the convention that makes no difference whether she wants to talk to them or not. The Russians are following due process and have every right to ask for access, by refusing access the British government are clearly in breach of the convention.
That would be a retrospective action (after the Russians had asked for access), and hence would not withstand legal scrutiny. Correct me if I am wrong, but only the Russians and Interpol can revoke a Russian passport. Imagine if any country can revoke passports of any other country, that would surely be a recipe for chaos.