I heard that there was a sefer written that has approximately 100 answers to the question of whether the Avot kept all of the Mitzvot. If anyone know about this and has the name of the sefer and author, please let me know. I presume it's out of print. But it would nonetheless be helpful information. Thank you.
I'm assuming you are looking for the seffer called נר למאה with a subtitle שבת האבות. However his hundred answers are focused on resolving the statement in Yoma 28 Avraham kept all the mitzvahs with the statement in Sanhedrin 58 a non-Jew who keeps Shabbos is punished by death, that along with what he says are many places in Shaas where we find only by Mattan Torah did the Avos stop being classified as a Ben Noach.
It's written by Rabbi Yerachmiel Zeltzer in Brooklyn N.Y. Inside the seffer is a number to call to buy the seffer. 718-851-7819. (I didn't try calling.)