How can the two noble verses be reconciled: "Indeed, Allah does not forgive association with Him, but He forgives what is less than that to whomever He wills" and "And indeed, I am a Perpetual Forgiver for whoever repents, believes, and does righteousness, and then is guided"?

There is no contradiction between the two verses. The first indicates Allah's promise of forgiveness to whoever repents and rectifies their deeds. The second implies that polytheism (shirk) will not be forgiven if its perpetrator dies upon it, but the polytheist will be forgiven if they embrace faith. As for sins less than shirk, Allah forgives them for whomever He wills among disobedient Muslims, and forgiveness is hoped for those who repent before the death rattle.