Reading about all this in English is a bit confusing. “Possession d'état de Français” has a more specific meaning: It means being treated as a French citizen by the French authorities. Concretely, you need something like having been granted a passport, voting, or serving in the French army. As far as I can tell from your question, nobody in your family has done anything like that since 1914 so the 50-year rule does apply.
Promoting French culture abroad, locating (or trying to locate) your family in France, etc. simply isn't relevant and does not establish the “apparent status of being French” as you call it. So I share the conclusion that you will not be able to make use of your French citizenship (oddly enough, the relevant provision of the law does not make any mention of any “loss” of citizenship, it just says that at some point you cannot “be allowed to prove“ that you are French).
In practice, this type of reasoning won't impress the Service de la nationalité des Français nés et établis hors de France and if you would apply for a certificat de nationalité, you would almost certainly attract a refusal. You would then need to go to court, which is always uncertain and costly under the best of circumstances and seems pointless if even your attorney does not believe in your case.