-Tsumugi lied about the participants being willing, meaning that Team Danganronpa takes in societal delinquents who "threaten the peace" and are used to provide entertainment via the killing games. Memory wiping is also consistently used in order to hide the fact that they would have recognized Rantaro and the Monokumarz.
-Majority of the audience leaving was simply staged by Team Danganronpa. The V3 survivors were ultimately detained, but later broke out and were rescued by a rebel group.
-Dangan Ronpa Is Dead is an American adaptation/spin-off of DR. The students chosen are actually biologically engineered testtube babies, who are specially created to both be cloned and be used for the killing game. (Cloning technology is not perfected, and while it would have been used in V3 it would have resulted in clones with short lifespans. As a result of this, the DRID students have been engineered from birth to be more compatible with the cloning techniques.)
-DRID students were also implanted with memory chips that would record their memories, allowing them to be studied and examined throughout their formalative years.
-They were raised within pods similar to the NWP, where they would go through their specially written and engineered backstories in order to create more "natural" characters.
-Following this, the DRID students would be sent to the Panopticon in order to record DRID.


What's the mastermind's motivation?
In order to incite anger and discourse within the world, likely causing something akin to WW3. This is to shatter the fake peace that relies on Killing Entertainment in order to revert to the "old days", back when life was what we would consider "normal".

Why does it ABSOLUTELY HAVE TO BE a Killing Game?
Through the means of a Killing Game that's televised as a long awaited sequel to DR53, the Mastermind's goal would be to hijack the game in order to obtain the tech used for it such as cloning and memory implantment/creation. By doing this, they would be able to create an army of soldiers capable of taking on majority of the world and launch nukes to incite warfare. The original intention of the game was to "revive Danganronpa", the mastermind simply killed the original mastermind (another fabricated character) and took over.

 By repeating the killing game various times, he would obtain the "survival instincts" needed in the memory data from the chips in order to implant them in the compatible clones that were created -- DRID was intended to be run through multiple times and to use the most interesting run, resulting in a large number of potential clones being available. By building up the survival instincts and abilities of the characters through the guise of multiple killing game reruns, the mastermind would slowly incorporate a means to allow the characters to "end" the killing game and expose the lies behind it.

The final trial involves the remaining characters discovering the truth, and partway through the clones with memories are re-introduced in order to further the discovery of the remaining mysteries. As the trial continues, the characters discuss the bombs that were gifted to Jak in the second chapter, which he lied about in order to use them to cause the reveal of the rest of the facility following the ch5 trial. This leads them to discovering that this was planned by the true mastermind who hijacked the game and took over as one of the characters: the PSGS Aquarist.

He reveals how he managed to fake himself -- he was part of the same group who had "insiders" who had the same goal of reverting the world as him, and was created as a "virus" to infect the character of the original PSGS Aquarist character. In short, he was made to be the "true" mastermind under the noses of Team Danganronpa. As the trial is underway, he reveals how the entire thing was being broadcast live, unlike the previous session, as his group had began their attack on Team DR HQ. He gives the cast a choice: Join him, and willingly destroy the world and live in peace afterwards, or have their personalities wiped and be turned into mindless drones. In order to demonstrate the power he has over them, he forcibly returns their memories of the previous killing games -- this results in them quickly growing exhausted, but they refuse to give in. They choose to defy the mastermind --

and before he can wipe them of their existances, the V3 survivors are able to break in and rescue the characters. This leads onto them escaping the facility which is promptly blown up, the mastermind killed during the event as the cast escapes. With the broadcast then ending, the characters discuss what to do now -- they decide that while his means were incorrect, the world does need to change. It may be a peaceful world, but peace brought upon by killing entertainment is wrong, and they seek to find a means to change the world itself.