Is refraining from desecrating the cross in the toilet, after being assailed by such a thought, considered an act of veneration for Christian rituals, or an act of disbelief, or apostasy, especially given the certain knowledge of the falsehood of their religion?

Obsessive thoughts have greatly afflicted you, and the cure lies in ignoring them and not paying them any attention. What happened to you does not constitute apostasy, so abandon these satanic thoughts and delusions. Seek refuge in Allah from Satan when these obsessive thoughts occur, and do not care about them. Know that you will be rewarded for your struggle against these obsessive thoughts.