The letter lead directly to the phrase "Natural Born Citizen" in the Constitution so I'm not sure why you are comparing an opinion of Jefferson to a request from a future Supreme Court Justice to the Father of our Country. It carries much more weight then your claims of the absence of questions about Arthur's parents' nationality status.
You can use “letters.” I prefer the language of the document itself. Unfortunately, the fuzzy language on citizenship does not help much; hence, we need to look to other souces including the Arthur precedent (which is against you) and the most recent court rulings since U.S v. Kim Wong Ark (which are also against you)