Great points. In some ways I think it seems to be attributable to the marketing/promotional/sensational benefit of individuals attaching history to their needs. In other words, if people are engaged in something (as an history author who wants to sell a book, a family member, historic property owner, teachers looking to connect with students, etc.), attaching the title “Founding Father” to a figure adds a sense of excitement and strength. As such, and as you mention many figures from that time are considered Founding Fathers, maybe it is in part to want to feel that sensationalized association with greatness.
As to whom should be considered the Founding Fathers, I would likely go with those whom were prominent figures from the 1st and 2nd Continental Congresses. They really dug in deep to not only summarize and give a single voice to the sentiment of the colonies, they also worked through the mechanisms of deciding to come together, design a plan for communication across colonies, develop a type of government, an army, currency, international relations (France, Spain), etc. and essentially build a country from the ground up.
If I had to start a definitive list of true Founding Fathers, I would say:
–George Washington – tremendous contributions to the army, leadership of the country, and character of the american spirit
–Ben Franklin – sound judgement across many areas of expertise, hard working/pioneering/inventive spirit, his direct contributions to founding a country (in Congress, on the Declaration, as ambassador in France, contributions to the post office, fire department, education, health)
–Thomas Jefferson – Balanced thinker who put country above self, drafted the Declaration of Independence (even while incorporating major concepts from earlier documents), leadership as ambassador to France, Secretary of State, Vice-President, & President