Many shuls in my community daven mincha on erev shabbos eight minutes before shkiyah, is it a problem if the chazzon’s shemoneh esrai goes past shkiyah?
This practice is not lechatchilah, and it is best to ensure that mincha is completed before shkiya.
However, bedieved one may rely on the ruling of the Mishnah Berurah, who writes that one can finish mincha until 13 minutes after shkiya (see Mishnah Berurah 233:8, 14).
One should nonetheless try to finish one’s own shemoneh esrei before shkiya.
This is all the more true on Erev Shabbos, when one must accept Shabbos early, before shkiya.
For those who cannot complete mincha until shkiya, Rav Pesach Eliyahu Falk advises that Shabbos should be accepted before mincha in thought alone, but not with a verbal statement.