The Manila hemp is derived from a species of banana originally from the Philippines, whose fibers are tough. The hemp is then used during the paper making process, similar to how Kraft paper uses wood pulp. So the mystery is solved, the manila envelope gets its name from the hemp in which it is made from.
No it will not poison you to drink a liquid that is directly cooled by dry ice. At normal pressures there may be some gaseous CO2 dissolved in the liquid giving it a mild carbonation. However, dry ice can be dangerous to bare skin, mouth, or GI tissue if someone swallows a medium to large pieces of dry ice.
Back in Pre-Revolutionary America when the song ‘Yankee Doodle’ was first popular, the singer was not referring to the pasta ‘macaroni’ in the line that reads ‘stuck a feather in his hat and called it macaroni.’ ‘Macaroni’ was a fancy (‘dandy’) style of Italian dress widely imitated in England at the time.