In the meantime: does the lecture series on the site, “Changes in Jewish Law” / “Authority and Change in Jewish Law,” give an initial direction that one can progress from?

Yes

Hello Rabbi:
First, I would like to thank the Rabbi for the enlightening lectures on the site and for the refreshing posts.
May you merit that your wellsprings continue to spread outward more and more.
For a long time I’ve been perplexed by the question of what level of authority the sages of the Talmud have (divine or otherwise), and what its source is.
From what I understood from the lecture series here on the site about change in Jewish law, someone who does not in principle accept the authority of the sages to enact ordinances and decrees violates the prohibition of “do not deviate.”
The problem is that, seemingly, the authority to enact ordinances and decrees does not really emerge from the passage about “do not deviate.”
A. The passage speaks about deciding doubtful cases; maybe that can be expanded to include deliberate interpretation and definition of the commandments, but absolutely not newly adding legislation.
B. The passage speaks about “the place that the Lord will choose,” and this repeats itself three times in the passage—it seems in the plain sense that this is specifically meant, and not just a generic description of where the sages usually are.
C. The passage speaks about “the judge who will be in those days,” and it is certainly very reasonable and expected that the judge of the generation should have authority to judge and lead his own generation. But to derive from here that a judge, or any human being, would have authority to enact ordinances and decrees for two thousand years into the future seems, on the face of it, really far-fetched.
It is hard for me to accept the argument that the sages received this by tradition, or interpreted the passage so expansively by virtue of the authority of interpretation granted to them, because this is not like places where the Torah was brief and did not explain a certain concept or law (such as what counts as labor, or what the fruit of a beautiful tree is), where one can readily accept that the explanation was transmitted orally or left to the sages’ definition. In our case, there is a very detailed and clear passage, and to expand it in this way is really to take it out of its context.

Again, thank you to the Rabbi, and sorry for the length.

Thank you.
This topic has already been discussed here in the past more than once, and you can search for it.
In general, Maimonides and Nachmanides disagreed about this (see his glosses to the first root). Nachmanides thinks like you, but even for him this is not because of interpretive considerations, but because of other difficulties (namely, that according to Maimonides every rabbinic law would come out having the status of a Torah-level law because of “do not deviate”). [Though in my opinion it is not certain that there is actually a dispute here. It may be that they both agree with the formulation you proposed.] It follows that according to all views there is no interpretive difficulty in this approach.
First, because ordinances and decrees can also be included in “if a matter is beyond you.” This too is instruction in how to serve God. Second, on logical grounds there must be an institution with authority to legislate, and it is very plausible that if there is already a Sanhedrin whose authority is to interpret and judge, it is that body that would also be authorized to legislate.
As for your other claims:
B. Indeed. The authority of “do not deviate” applies only to the Sanhedrin in the Chamber of Hewn Stone. The Talmud received its authority through the agreement of all the sages of Israel, and after it there is no longer any “do not deviate” authority (except according to the very puzzling view of Sefer HaChinukh).
C. Obviously, this authority applies only to the judge (the Sanhedrin) of its own days. There is no “do not deviate” authority regarding future generations. Rather, the sages of each generation can reaffirm the existing law (and that is indeed what they do). By the way, this is exactly like state law as well: even an ancient law binds us as long as it has not been changed, even though that old parliament was not elected by us.
In my opinion, the sages did not receive any tradition on this matter. It is a decision of the sages of the generations not to disagree with the Talmud.
By the way, even when the Torah elaborates and spells things out, there is still room for interpretive expositions that add, and there are examples of this.