Doubts are growing over Guyana's pick of a little-known U.S. startup to craft and develop projects to monetize its vast untapped natural gas resources that could cost up to $30 billion. Year-old Fulcrum LNG faces financing hurdles that could derail its selection. Ultimately, the South American nation may end up relying on a consortium led by Exxon Mobil, which controls all the production in the new energy hotspot…