Attorney General Merrick Garland is the unnamed official whose sworn testimony before Congress is being challenged in a bombshell letter from an IRS whistleblower’s attorney that also alleges a coverup in the Hunter Biden criminal investigation, The Post has learned. Attorney Mark Lytle wrote Wednesday that the longtime IRS employee wants to provide information to congressional leaders to “contradict sworn testimony to Congress by a senior political appointee” — Garland —and also to detail “preferential treatment” in the criminal probe of the first son. The whistleblower already has made disclosures to the inspectors general of the Treasury and Justice departments....