How can one reconcile the Hadith, "If his Islam is good, then he is not held accountable, but if not, then he is held accountable," with the Hadith, "Islam obliterates what came before it," and are they contradictory?

The two Hadiths do not contradict each other. The Hadith of Ibn Mas'ud means that whoever excels in Islam by entering it with a true entrance, his Islam demolishes what came before it. And whoever acts wickedly in it with hypocrisy, is held accountable for both sins. What is meant by excellence is the reality of Islam, and by wickedness, it is hypocrisy. Some scholars are of the view that excellence is dying upon Islam, and wickedness is dying upon disbelief. Accordingly, whoever embraces Islam truly and dies upon it, his Islam demolishes the sins that came before it. And whoever disbelieves after his faith or embraces Islam hypocritically, is held accountable for both sins.