The pathogenesis of ectopic gastric mucosa is a matter of debate between a congenital-embryological-and an acquired origin-consecutive to a gastroesophageal reflux (GER) like Barrett's esophagus -, the most commonly accepted theory being that these areas are embryological remnants.
Standard size latex filled helium balloons stay afloat for approx 8 - 12 hours, metallic helium filled balloons float for 2-5 days. I have found that a 36 latex helium filled balloon can stay afloat for a number of days due to the thickness of the balloon and the massive quantity of helium.
Helium balloons rise in air because Helium is lighter than air. Helium rises because it is less dense than the air. If it is less dense then it will rise above the air until it meets a gas lighter than it. There are other gases lighter than air. You should experiment with other gases inside balloons.