Can someone make Yom Tov early by making kiddush after plag hamincha on 7th day of Pesach? Also, in chutz la’aretz can one make kiddush after plag hamincha on the 8th day of Pesach if it falls on Friday night?
It is fine to make kiddush early on the seventh day of Pesach, and the same will apply for making kiddush for the eighth day of Pesach which falls on Shabbos.
There is in any case a mitzvah of Tosefes Shabbos, so that one must add some amount of Yom Tov to the coming Shabbos. Making kiddush early extends the Tosefes Shabbos, and this is fine.