Does Barrett's oesophagus always cause oesophageal cancer?

No. Barrett's esophagus is one of the complications of GERD. It is characterized by the replacement of the normal stratified squamous epithelium lining of the esophagus by simple columnar epithelium with goblet cells About 10% of people with chronic symptoms of GERD develop Barrett's esophagus.Patients with Barrett's esophagus do have an increase the risk of developing esophageal adenocarcinoma but the complication is rare with less than 1% of patients developing this particular cancer.
No. Barrett's esophagus increases the risk slightly. Visit this site for more info in this topic.
No. Only a few percent. Your physician knows proper surveillance based on appearance and biopsy results.