Can A TRUE Jew Be A Kofer?/ Moses

The Rambam writes in his letter to the Jews of Yemen - אגרת תימן - that anybody who doubts the veracity of מעמד הר סיני and/or the prophecy of Moses [known by many Jews in places like Monsey and Brooklyn as "Moshe". I have fond memories להבבבדיילללל of the late "Moses Malone", called by his Jewish accountant 'Moishe', center for the 76ers who famously chanted "fo, fo" predicting the number of games the Sixers would need to win each of the three series they would play on the way to a championship. They nearly accomplished this prediction of a sweep of all three rounds, only losing one game (to Milwaukee in the Eastern Conference Finals) en route to the championship. So no prophecy for THAT Moses. Some Caucasians pronounce that word as "four". There was a popular poster of him with the caption that Moses will lead the Sixers to the Promised Land. He didn't. They stayed in Philly and never made it to Israel😊😊] was simply not at הר סיני when the Torah was given nor is any denier descended from people who witnessed it.   

וכבר הבטיחנו הבורא יתברך כאדם הנכנס ערב לחברו ודי לנו בערבותו והודיענו שכל מי שעמד על הר סיני שהם מאמינים בנבואות משה רבינו בכל מה שבא על ידו הם ובניהם ובני בניהם עד עולם שכן אמר הקב"ה יתברך (שמות י''ט ט') הנה אנכי בא אליך בעב הענן בעבור ישמע העם בדברי עמך וגם בך יאמינו לעולם לפיכך יש לדעת שכל מי שנטה מדרך הדת הנתונה במעמד ההוא שאינו מזרע האנשים ההם וכן אמרו רבותינו ז"ל על כל המסתפק בנבואה לא עמדו אבותיו על הר סיני המקום יצילנו ויצילכם מן הספק וירחיק ממנו ומכם המחשבות המביאות לידי ספק ולידי מכשול. 

We have been given adequate divine assurance that not only did all the persons who were present at the Sinaitic Revelation believe in the prophecy of Moshe and in his Law, but that their descendants likewise would do so, until the end of time, as it is written, "Lo [great word!!], I come unto you in a thick cloud, that the people may hear when I speak with you, and may also believe you forever." (Exodus 10:9). Consequently it is manifest that he who spurns the religion that was revealed at that event, is not an offspring of the folk who witnessed it. For our sages of blessed memory have insisted that they who doubt the divine message are not scions of the race that were present on Mount Sinai. May God guard us and you from doubt, and banish from our midst confusion, suspicion, which lead to it. 

Now the Yerushalmi [Sanhedrin 1-1] say that Korach said that there is no תורה מן השמים and Moshe is not a Navi. So he denied both תורה מן השמים and נבואת משה. But Korach was clearly at הר סיני. So according to the Rambam - how is this Kfira possible? The Rambam HIMSELF writes [Yesodei HaTorah 8-1] that Korach denied the נבואה of Moshe Rabbeinu??

Also, how can one receive reward for believing? It is an automatic?!

Another question - How can you have two siblings, one a believer and one not?? Where their forefathers at Sinai or not? The believing brother would seem to indicate that yes while the non-believer proves that not.