MB3 193b: Rabbinical Prohibitions During Twilight on Shabbos (342:1)

Some Shabbos-related Rabbinic decrees can be done during בין השמשות for the purpose of a mitzvah. This only applies when one hasn't yet accepted Shabbos (or if one lives in a town with one minyan and they haven't yet accepted Shabbos).

Do the same leniencies apply during בין השמשות at the end of Shabbos?

אמירה לנכרי, climbing on a tree and swimming for the purpose of a mitzvah.

Making an Eruv Techumin during בין השמשות.