The perpetrators needed to make sure that people, including experts, would be fooled for enough period of time so that the media or politicians, or experts or other people in positions of power and/or influence would not be able to revert the story about a terrorist attack. Say one or two days. After that there was no turning back and a cover-up HAD to be done because the public could not be told the truth without it having grave consequences for the entire society.
This means that the jet engine was probably of the CORRECT kind. Am I making a flip-flop here? You betcha! I'm faster than lightning.
So, then how to prove that the jet engine core was planted? Easy. Simply check the serial numbers on the different parts of the jet engine and the landing gear, and compare that to the actual listings for Flight 175 and Flight 11. As I understand it, all parts of an airliner must have a serial number or something to that effect.