I believe I heard that there is an opinion that it is sufficient to make KIDDUSH after nightfall the eve of Shavuos even though the prevailing custom is apparently to wait to daven Maariv after nightfall the first night of Shavuos. Being that I am situated with my parents, I would like to know if this is indeed a valid opinion. And if it is a valid opinion, I would also like to know if I can daven Maariv early i.e. BEFORE nightfall the first night of Yom Tov. All the best and thank you very much in advance.

ב"ה Shalom There are indeed such opinions. (של"ה ריש מסכת שבועות, סידור יעב"ץ, ) So you may Daven Maariv before nightfall. Just remember to repeat Shema after nightfall. (See Piskei Teshuvot 494) All the best