:- Please assume a less dismissive tone. The business  about how a PhD says there is no constitution, because he would help you distinguish between two classes of constitution which are generally understood by students of government and political science  is called, in logic and in rhetoric, reducing your opponents argument to absurdity, reductio ad absurdum. In high school and college debate, you will have points deducted from your side for using either faulty logic or an oratorical trick. Debate judges are neither amused nor impressed, unlike the reporters passing along candidate-employee recaps after a presidential debate in the spin room for national television instant analysis. Here, it just means you have violated one of the pillars of wikidom, collegiality.  
 :- Sources abound establishing Magna Carta as is a written constitution for the English in 1215, I believe there are earlier such 'written constitutions' elsewhere in Europe. For instance, the golden age of the Republic of Florence begins with the constitution of 1343, earlier "Captains" as you properly call them, were elected in Florence as a legislative body in the mid-1200s. See William R. Everdell, "The end of kings: the history of republics and republicans", ISBN 978-0-226-22482-4, pp.87-107. 
 :- There is no need to rehearse either the history of written constitutions, nor the history of republics in the U.S. Constitution article introduction.  A couple years ago, I think an earlier version of the section on the 'origins of the constitution' had my contribution referring to the Republic of Venice as a model, quoting from the Federalist Papers, but I think that that was overwritten months ago. Regardless, INTRODUCTIONS require very concise, WP:SUMMARY style. 
 :- I like San Marino. San Marino made one of my heros, Abraham Lincoln, a citizen during his lifetime. San Marino has published beautiful stamps since 1877. When I was a boy, I had a 1920s commemorative stamp in my collection showing the three fortresses on the mountain. My family has great stories of Italy and Italians. My grandparents saw Etna erupt in 1928. The first place I ever saw city streets washed clean in the morning by fire hydrants was in Milan in the 1960s. San Marino is a wonderful place, the architecture of city hall square alone is worth the visit. Can't we all get along?