Is merely writing the name of the film "THE CITY OF GOD" to search for it considered disbelief (kufr)? And is mocking the Christian doctrine of the Trinity by performing the sign of the Trinity considered something (sinful)? And is ignorance an excuse in both cases?

Praise Allah for shunning idle talk. There is no room for takfir (declaring someone an apostate) in what you mentioned, nor for your heart being expanded by kufr (disbelief); for Allah Almighty says: "But whoever expands his breast with disbelief - upon them is wrath from Allah, and for them is a great punishment. That is because they preferred the worldly life over the Hereafter and that Allah does not guide the disbelieving people." It is not permissible to imitate them in making the sign of the Trinity. Turn away from the whispers of takfir, and we advise you to constantly supplicate and humble yourself, to distract yourself from these whispers, and to consult a trusted psychiatrist.