1. I read it, and I do in fact agree that according to the plain meaning of the verses, the supreme legislative institution has the authority to rule on new difficulties (“if a matter is too difficult…”).

But! Those verses have nothing whatsoever to do with adding commandments — enactments and decrees.

So I am asking: where did the Sages find the ability to institute enactments?!
If it is based on some other unrelated verse, from which they derived a mere textual support, then that too is not a power given to them by the verses, because that is circular.

As for the serious logical error in the Rabbi’s comparison between the Holy One, blessed be He, and the Sages:
I just wanted to say that unlike the fact that the Holy One, blessed be He, can establish commandments etc., and any person can choose not to comply, the Sages are forbidden to do something that is not part of the Torah. After all, it is written: do not add.

If they have the authority to interpret, it stands to reason that they also have the authority to legislate when necessary. That is simple logic. Just as the Supreme Court was not given authority to legislate, and yet it does so anyway (judicial legislation — for example, Aharon Barak’s constitutional revolution). Those who criticize it merely for judicial legislation itself are mistaken. There is certainly room to criticize the degree, of course.
By the way, “if a matter is too difficult” does not necessarily deal with interpreting the Torah, but with any case in which there is a legal matter and people do not know what to do, or a case between people. That is why it also says, “according to what they instruct you,” and that already includes enactments.
And beyond all that, the Sages have the power of “it is in his power” (see the Rosh, Bava Batra 34, where it is stronger than migo). It is in their power to present things as interpretation of the Torah, and then it would be binding, so they also have the power to legislate a new law.

As for my “serious logical error”: first, who says that “do not add” refers to the Sages? (As is well known, the medieval authorities (Rishonim) disagreed about this. See Rashba and Tosafot on Rosh Hashanah 16b.) Second, I am speaking about the fundamental authority. You asked how they have authority without a source, not how their authority fits with the prohibition of adding to the Torah. To your question I answered with a comparison to the Holy One, blessed be He, who also has authority without any source.
The second question was already asked by those medieval authorities (Rishonim) and others as well (for example, Maimonides in the laws of Rebels writes that this really is said also about the Sages, but when they legislate a new rabbinic law they must indicate that it is not of Torah origin, and in that way the prohibition of adding is avoided).

1. Did they receive permission to make enactments from the Oral Torah, or was it apparently just independent reasoning?
[2. The power of “it is in his power” is also circular, so the difficulty returns there as well.]

1. I don’t know. But either way, they have authority.
2. You are treating “it is in his power” as a law the Sages established, but that is not an enactment; it is a logical inference. You can of course disagree with it, but there is no circularity here.