A Jewish person, by maternal line, by heritage, by name, by history, ceases to be a Jewish person? By changing their beliefs they obliterate their maternal line, their heritage, their history? My friend, that person by definition and by birth is a descendant of true Jews, except they do not practice Judaism. Perhaps they ought to be called non-practicing Jews. That is what Catholics are called when they no longer believe as their church does.
Using the Socratic method of reasoning would lead us to be in contradiction with our own premises. By your last definition, those that carry names of possible sephardic origen, BY IMPLICATION, have a Jewish origen (although practicing Judaism would be foreign to them ) and therefore they would be "Jewish". As you can see the implications arising by mentioning to some their names are those of Sephardic Jews are far reaching and can be confusing. The distinction of being "Jewish" by birth and that of being "Jewish " by belief is not normally encountered by most people and thus can be confusing.
At best one can say perhaps that some of those that carry surnames of possible sephardic origen, may at one time have had ancestry of Jewish heritage, for genealogical purposes.
Good luck with your work , my friend. It is both informative and necessary.