Is continuing to watch a movie that contains a "t" shaped like a cross, and not deleting it from the device, considered apostasy from Islam merely by being pleased with the symbol of the Christians, or is it merely a sin that does not amount to apostasy? And is a mental (heartfelt) statement, "no matter how sinful it may be," considered apostasy?

Not every two intersecting lines form a cross. A cross is what resembles a crucified human being. The letter "t" has no relation to the cross. Allah, the Almighty, has forgiven this nation for what they merely think, as long as they do not act upon it or speak of it. It is obligatory to seek treatment from a trustworthy Muslim doctor, for Allah has not sent down any disease without also sending down a cure for it.