How can the two noble verses be reconciled: "Indeed, Allah does not forgive association with Him, but He forgives anything less than that for whoever He wills" and "And whoever kills a believer intentionally - his recompense is Hell, wherein he will abide eternally"?

There is no contradiction between the verses. Allah's statement, "Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills," shows that anything less than polytheism is subject to Allah's will. He may forgive it as a grace from Him, or He may punish the perpetrator according to the extent of their disobedience. As for the polytheist, Allah will not forgive him; rather, he will abide eternally in Hell.

As for the verse, "But whoever kills a believer intentionally - his recompense is Hell, wherein he will abide eternally, and Allah has become angry with him and has cursed him and has prepared for him a great punishment," it means that this is his recompense if Allah chooses to punish him, but He may also pardon him. The eternal abiding here is a temporary one, not like the eternal abiding of disbelievers, which is perpetual. Disobedient Muslims are subject to Allah's will; if He wills, He will forgive them, and if He wills, He will punish them for a period then release them from Hell through intercession or by His grace. None will remain in Hell except the disbelievers.