Question

Hello Rabbi,
Last Friday (the seventh day of the festival) I arrived in the afternoon at a synagogue where a Messiah’s Feast (of Chabad) was taking place. One of the Chabad guys tried to persuade me to eat matzah, and after some back-and-forth I remembered that I had in any case already accepted upon myself the addition to the Sabbath on the way to the synagogue. With a feeling of gratitude to God, I informed the Chabad fellow that I could not eat, since I had already brought in the Sabbath and was obligated in kiddush. He, for his part, argued that the addition to the Sabbath is not Torah-level, and that for the sake of a commandment need (he includes the Messiah’s Feast in that category) I am exempt from the Sabbath prohibitions of this addition. Afterward I saw that the addition to the Sabbath is Torah-level, and that in tractate Rosh Hashanah they even derive it from the Torah (I think by analogy), but this argument of mine also met with his response that even though the commandment itself is from the Torah, the prohibitions that apply to me during it are not Torah-level. His first reason was to point to the way the halakhic decisors relate to the time of the addition to the Sabbath and the various leniencies they propose when there is a need, and his second reason was that the Sabbath prohibitions apply by Torah law at their fixed time and in an objective way, and I cannot choose when to make them apply.
I would be glad to know the Rabbi’s opinion on the matter.