Is the prayer of the congregant valid if he interrupts his recitation of Al-Fatiha and bows with the Imam, due to his inability to pronounce “Al-Dalleen” correctly, and does the Imam bear that on his behalf?

What you are experiencing of difficulty and distress in pronouncing the letter Dhād in the word "al-Ḍāllīn" is from Waswasa (whisperings from Satan), and you should not pay attention to it. It is sufficient for you to pronounce the letter Dhād to the best of your ability, and your prayer is valid, even if the Imam bows while you are repeating the word. If you were to omit the word "al-Ḍāllīn" due to Waswasa and the difficulty of pronunciation, your prayer would not be invalidated, even though Al-Fatiha is a pillar for the one praying behind an Imam.