Which is more preponderant to be adopted: Divorce during the time of the Prophet, may Allah bless him and grant him peace, and Abu Bakr and Umar, may Allah be pleased with them both, or Umar's independent reasoning (ijtihad) regarding the occurrence of triple divorce, or that there is latitude in the disagreement?

The relied-upon opinion among us regarding the issue of divorce is the view of the majority (جمهور) that divorce takes effect. Your situation is one of two: if you have religious knowledge, then act upon what you deem most sound (ترجح); otherwise, seek a fatwa from someone you trust and act upon their ruling. It is not permissible to choose the easiest of the imams' opinions with the intention of seeking out dispensations (تتبع الرخص), although some scholars have permitted taking a dispensation to alleviate hardship (دفع الحرج). As for the narration from Ibn Abbas concerning the triple divorce counting as one, scholars have discussed it and countered it with other evidence.