Is it correct for the doer of "yashāʾ" (wills) in His, the Almighty’s, saying: "Indeed, Allah leads astray whom He wills" to refer back to "man" (whom), so that the meaning becomes: "He leads astray whom wills misguidance, and guides whom wills guidance"?

The agent of will is Allah, the Almighty. None has differed from this except Al-Zamakhshari in his invalid Mu'tazilite issues. This is evidenced by the words of Allah the Almighty, relating from His prophet Moses (peace be upon him): "It is none but Your trial by which You let astray whom You will and guide whom You will," and also His saying: "And if Allah had willed, He would have made you one nation, but He lets astray whom He wills and guides whom He wills." The meaning is that if Allah had willed, He would have gathered them upon guidance, but His wisdom necessitated otherwise, so He led astray whom He willed to lead astray and guided whom He willed, as explained by Al-Qurtubi and Al-Alusi.