"Expound and Receive Reward" Expound What?

There is an opinion (R' Eliezer - Sanhedrin 71a) that there never was and never will be an Ir Hanidachas, a subverted city. According to this opinion, the reason for this commandment is:

It's true, we earn great reward for learning Torah. But if there wan't enough Torah without Dueteronomy 13:13-19 why was this particular Mitzvah added. After all, if a Mitzvah needs to be designed for a scenario that will never happened, there are lots of scenarios that will never happen to choose from. Why was this scenario chosen? On the other hand if the goal is just to increase the amount of Torah, why not add on additional laws to some other Mitzvah from the Torah.

This question suggests that there is a specific lesson that we are supposed to learn from an Ir Hanidachas. It is a lesson that an Ir Hanidachas was specially designed to teach. In other words I am suggesting that the words:

refer to a specific Drasha, a specific lesson we derive from Ir Hanidachas. Similarly for a Ben Sorer, a rebellious son where the same phrase is used (Sanhedrin 71a) we are to receive reward from learning a particular lesson from a Ben Sorer. The lesson from a Ben Sorer is that a kid steals from his parents, goes to a bar and gets stoned, we whip him and if he does it again we get him drunk and stone him permanently. Maybe this lesson will come to mind the next time a kid thinks "What's the worst that could happen to me if I ....."

What lesson can we learn from an Ir Hanidachas? Here we have a

A) city in Israel (Sanhedrin 16b),

B) that worships idols and

C) we are commanded to smite the city with the sword and make all it's possessions cheirem; anything alive is killed by the sword and the rest is burned.

D) Afterwards it remains a permanent ruin never to be rebuilt

This is strikingly reminiscent of the command to smite the idolatrous Canaanite cities in Israel and in some cases (Joshuah 6:17,24,26 Numbers 21:3) condemn them to cheirem, never to be rebuilt.

The lesson is, if you ever dare succumb to idolatry, be prepared to receive the punishment you are about to afflict on the Canaanite for the exact same sin.

This Mitzvah is a concretization of the value expressed in Deuteronomy 9:4-5

Without the oath to your fathers, you have no more connection to the land of Israel than the Canaanites do. The Canaanites need to be driven out because of their wickedness regardless of your arrival. Ir hanidachas teaches us that similarly we must wage war against Jewish cities in Israel when their wickedness warrants it. Whether its Jewish or not, a city must be destroyed if it commits idolatry. Being Jewish, provides no protection for an idolatrous city to remain in Israel, just as 9:5 teaches that being Jewish, without bris avos, provides no stronger claim to the land of Israel.

However, is an Ir Hanidachas actually reminiscent of the conquest of Canaan? If you witnessed the conquest of Jericho and then witnessed the procedure of an Ir Haniachas would it occur to you that there was a parallel between the two procedures? Absolutely not. The conquest of Cannan involved bloody battles while the procedure of an Ir Hanidachas involved court cases. Very, very long and repetitive court cases with tedious cross examinations (13:15). These were followed by a retrial before the Supreme Court (Mishneh Torah, Idolatry 4:6) to ensure a plethora of technical criteria were met.

How do we account for the fact that even though an Ir Hanidachas is so different from an attack on a Canaanite city in practice, yet the Torah's description of them is so similar?

Perhaps, despite the fact that it's technically inaccurate, we are still supposed to draw the lesson from the parallel between the conquest of Canaan and an Ir Hanidachas; that if you ever dare succumb to idolatry, be prepared to receive the punishment you are about to afflict on the Canaanite for the exact same sin. Put in different terms, perhaps we are supposed to draw the literary parallel (or the Halachik parallel) between the two ideas even though in actual practice they are dissimilar.

This insight, that the Torah wants us to derive lessons from parallel descriptions even if the similarity is technically incorrect, may be a principle or a tool that we can apply to the rest of Torah. This may be an additional lesson that Ir Hanidachas was designed to teach us.