ir ha'nidachas -- a sum greater than its parts

The Ramban in this upcoming week's parsha quotes a machlokes between himself and R' Sadya Gaon whether a beis din of 23 is required for the kabbalas eidus in capital cases, or whether the b"d of 23 is required just for the psak and punishment, but the kabbalas eidus can even be done in the b"d of 3.  Do you need the same # of judges for kabbalas eidus as you need to try the case?  

The Minchas Chinuch raises a question on Ramban from the parsha of ir ha'nidachas that we read this past week.  When there are number of cases of idolatry in the city, b"d first sits in judgment of each case as if it was a din on an individual.  After they figure out that it's not just isolated individuals, but it's the majority of the city that is guilty of idolatry, the case is moved to b"d ha'gadol of 71 (Sanhedrin 112) to judge as an ir ha'nidachas.  If kabbalas eidus always has to be done in a court that has the same # of judges as those that will try the case, why is the kabbalas ha'eidus that was originally done in courts of 23 sufficient for ir ha'nidachas?  Shouldn't it require 71 dayanim from the get-go? 

Yesh lachkor whether a city is just a collection of individuals, or whether a city is a new collective unit?  Is the whole greater than the sum of its parts (same chakira with respect to definition of a tzibur)?  

The M.C. treats ir ha'nidachas as a city where, if there were 100 individuals living there, for example, we have 51 dinei torah going on against the population, and those case have to be transferred to a higher court of b"d ha'gadol.  But it could be that what is transferred to b"d ha'gadol is not 51 cases, but rather 1 case, a case against the entity of the city.  (I'm not sure, but it sounds like this is what R' Shmuel Rozovski quoted here means.)  

In the parsha sheet based on the chiddushim of R' Avrohom Genochovski z"l this past week there was a discussion of a question raised by the Chasam Sofer (I wrote about it 10+ years ago as well): since the children as well as the adults of of an ir ha'nidachat are killed, why isn't the testimony against them considered עדות שׁאי אתה יכול להזימה?  After all, we cannot inflict parallel punishment on an eid zomem and kill his children, because, as we learn in first sugya in Makos, לו - ולו לזרעו?  

Here too, the answer could be that the eidim who come to testify are testifying against individual idolaters.  The reason the children of an ir ha'nidachas are killed is because beis din ha'gadol has its din torah against the city as whole -- a whole greater than the sum of dinei torah against its individuals -- and the punishment that applies to the city extends to the children living there as well.

We once discussed the shitas haRambam that seems to say that an ir ha'nidachas can do teshuvah.  Since when can a person simply do teshuvah to get out of misas b"d?  It could be that what the Rambam means is that the city as a whole loses its status of ir ha'nidachas if enough people do teshuvah, but the individual idolaters who comprise most of the city will still be chayav misas b"d for their actions.