Is the definite article "al-" in the word "Shaytan" (Satan) in the Almighty's saying, "So when you recite the Qur'an, seek refuge in Allah from Satan, the expelled [from His mercy]," for a mental referent, thus limited to Iblis, or is it for the generic class, thus encompassing all devils among jinn and humans?

The "al-" in "al-Shaytan" (the Devil/Satan) can be for generic reference (jins) or for specific reference (ʿahd). If it is for generic reference, then devils are many, including those seen and unseen. If it is for specific reference, then all sins occur with the approval or instigation of Iblis, and one who approves is in the same position as one who performs the act.