You will not add to it or subtract from it.

Hello. This question has already been asked by the Rishonim, and there is disagreement among them on this matter (see Toss and Rashba, 16b, and Rambam at the beginning of the Laws of Mary). As a principle, some of the first scholars understand that the prohibition "do not add" does not apply to the sages at all, since the Torah itself commands them to "keep watch for my watch" and to interpret the Torah and to correct and issue new laws. Others argue that what the sages do is not included within the scope of the prohibition (for example, because the prohibition is only to add details and not to add laws. Or: the prohibition is to add laws from the Torah and not the laws of the rabbis that are declared and presented as such. This is how Maimonides believes at the beginning of the Laws of Mary). Beyond that, you must distinguish between an interpretation of the laws of the Torah and regulations and decrees that are legislation that creates the laws of the rabbis. For example, the Amidah prayer that you mentioned, according to the Maimonides (who believes that prayer is from the Torah) is only an interpretation (which determines the form of the prayer) or an addition from the rabbis (and from the Torah every prayer is in order). And for the Maimonides, all prayer is an innovation of the Sages, and within its framework they determined to say 18 blessings. A Hanukkah candle is, of course, a rabbinical regulation, and as long as they declare that it is not a mitzvah from the Torah, there is no "don't add" to it. I didn't understand what contradiction you found between "do not forsake" and "do not add to." Beyond that, I also disagree that the accepted interpretation of "do not forsake" is taken out of context. Absolutely not (see my comments in response to the igod videos).