By David Stanway SHANGHAI (Reuters) - Russia's invasion of Ukraine and the imposition of tough sanctions on Moscow has put landlocked Mongolia in a tight spot economically and diplomatically, and experts warn that its delicate balancing act between East and West could be upset. On Tuesday, a small group of protesters gathered on Ulaanbaatar's Sukhbaatar Square to call for an end to hostilities in Ukraine, only to be confronted by residents worried about antagonising Russia. The stand-off reflected the faultlines in a country that underwent a democratic transition in 1990 following decades of S...