The 14th Amendment was intended to keep former Confederates out of government. The people who wrote it had no intention of putting former Confederates on trial.
Right, but 14A has only ever been used to disqualify two categories of people - public officials of the Confederacy and people convicted of an appropriate crime (such as the Espionage Act or charges related to Jan 6).
Trump is neither, so he's going to challenge being disqualified by anything less on due process grounds. 14A is vague on that. Which ends with SCOTUS essentially deciding what due process should be, likely by looking at how it's been used historically.