What is the ruling on one who swore to abandon unlawful acts, stipulating two months of fasting if he committed them, then committed them and repented? And is it obligatory for the fasting of expiation to be consecutive?

If someone swears by Allah to abandon something forbidden and then breaks their oath, they incur the sin of committing the forbidden act and of breaking the oath. You are obligated to perform the expiation for breaking an oath, which is fasting for two months. If you are unable to do so, then the expiation for an oath mentioned in verse 89 of Surah Al-Ma'idah applies. However, if the oath was not by Allah, but rather by obligating oneself to fast for two months to prevent oneself from committing a forbidden act (an oath of obstinacy - nadhr al-lajaj), then you have the choice between fasting for two months or performing the expiation for an oath. According to the majority of scholars, continuous fasting is not required unless you intended continuity when you made the oath.