Is it obligatory to adopt the opinion of Ibn Abbas, may Allah be pleased with them both, in matters of Fiqh (Islamic jurisprudence) and Tafsir (exegesis) and not to contradict it, based on the Prophet's prayer for him, "O Allah, grant him understanding in the religion and teach him interpretation"?

The Prophet, peace and blessings be upon him, prayed for Ibn Abbas to be granted understanding in religion and knowledge of the Quran. This was indeed fulfilled, as Ibn Abbas became one of the most knowledgeable scholars and exegetes among the Companions. However, these hadiths do not establish infallibility for Ibn Abbas's sayings, because he was not prayed for to be infallible from error. A scholar occasionally not arriving at the truth does not remove him from the ranks of scholars. Fiqh (understanding) does not mean that a scholar is correct in all his pronouncements; all jurists are sometimes correct and sometimes mistaken. Furthermore, there are reports regarding the virtues of other Companions that are greater than what has been reported about Ibn Abbas, and this has not necessitated any of them being infallible. Indeed, Ibn Abbas and other Companions differed with the Rightly Guided Caliphs on some matters. Interpreting "wisdom" as being correct in speech is merely one of multiple interpretations, and the more preponderant view is that it means understanding of the Quran. It is unanimously agreed upon by the Ahl al-Sunnah that no one after the Prophet, peace and blessings be upon him, is infallible. Therefore, scholars treat the sayings of Ibn Abbas like the sayings of any other Companion.