Is it permissible to swear falsely upon the Quran to دفع (ward off/repel) an accusation of theft, when the one swearing did not steal, but was aware of the thief and close to the scene of the theft?

If the intention of the one who made you swear was not contrary to the apparent meaning of the لفظ (wording), then nothing is binding upon you as long as you did not steal. However, if you were involved in the theft, then this oath of yours is an oath of Ghamus (a false oath sworn knowingly), and you must repent from it, and from betrayal and deception.