Is departing from Islam and being held accountable for intrusive thoughts about the Divine, along with unanswered prayers, a reality, especially with feelings of frustration and worry?

Allah does not hold one accountable for whispers (waswas) and thoughts of the self unless they are acted upon or spoken. A servant's dislike of whispers of disbelief is a sign of the soundness of his faith. To deal with these whispers, one must ignore them and not pay attention to them, while having a good opinion of Allah and supplicating to Him to remove them from you. It is important to seek refuge in Allah when these whispers occur, and to say "I believe in Allah" when asked, "Who created Allah?" It is also recommended to say: ﴿He is the First and the Last, the Manifest and the Hidden, and He is, of all things, knowing.﴾ (Al-Hadid: 3) when experiencing a whisper concerning faith. The chaining of the devils in Ramadan (definition) means their whispers weaken, not that they completely cease to exist.