If someone accepts shabbos after pelag hamincha, can he make kiddush and eat, and pray maariv later (after tzeis)? Can this be done lechatchillah? Is his kiddush, al netilas yadayim, and birchas hamazon counted towards the 100 brachos for friday or shabbos?
Yes, this arrangement is fine, and can be done lechatchillah.
Sources: According to a number of rishonim, including ra’ah and ritva, the optimal way to accept tosefes shabbos is by making kiddush (or by davening). This clearly indicates that it is fine to make kiddush and to eat the meal before davening. The single source that mentions this should not be done is Maaseh Rav, citing from the Vilna Gaon, but this ruling is not cited in other halachic texts, and many families with young children make kiddush early in order to allow the children to participate, and daven after or half-way through the meal. The berachos count towards the 100 berachos of Shabbos.