To me, it just breaks down into some syllogisms.
If Trump ORDERED Comey to drop the investigation, it's a crime
Trump ASKED Comey to drop the investigation (by Comey's own testimony)
Therefore, it's not a crime
If a President is proven to have committed a crime, he can be impeached
Trump has NOT been proven to commit a crime re: Comey investigation (yet)
Therefore, Trump can't be impeached (for this)