TIM COCKS and CARIEN DUPLESSIS SOUTH Africa and its neighbours were at the centre of a tussle for influence this week when top Russian and U.S. officials visited, offering a rare moment of leverage for governments on a continent more used to being buffeted by events than wooed. With a war in Europe pitting invading Russian forces against Ukraine's army supplied with Western arms, Russian Foreign Minister Sergei Lavrov and U.S. Treasury Secretary Janet Yellen were both on the hunt for broader international support. For the countries of southern Africa, which maintain strong ideological and historical sympathies for Russia but…