Is it permissible to continue reciting specific surahs in the Duha prayer—namely: (هل أتى على الإنسان) [Al-Insan] in the first two rak'ahs, Surat Ad-Duha in the third rak'ah, and Surat Al-Ikhlas in the fourth rak'ah—or is that considered an innovation?

It is better not to consistently recite specific surahs in the Duha prayer, such as always reciting Surat Ad-Duha, lest the worshipper believes that it is obligatory, and because there is no origin or evidence for that. A Muslim may recite any surahs or verses in the Duha prayer, and it is better to vary the recitation. As for the number of rak'ahs for the Duha prayer, the minimum is two rak'ahs, and it is better to add two rak'ahs at a time, with a taslim after every two rak'ahs, based on the saying of the Prophet, peace and blessings be upon him: "The prayer of the night and day is two by two."