Is the prayer valid behind an Imam who does not pronounce some letters clearly in his recitation, such as the letter "kaf" in "You alone we worship and You alone we ask for help" (إياك نعبد وإياك نستعين)?

The Imam's fast recitation of the Quran contravenes the guidance of the Prophet, peace and blessings be upon him, and prevents contemplation of the Quran. Whoever does not contemplate the Quran is blameworthy. Nevertheless, praying behind such an Imam is valid as long as you are not certain that he omits letters from Al-Fatihah, because the default assumption is that his recitation is correct, and this default is not abandoned based merely on doubt.