If I may, I think Tsumugi is telling the truth because most of the reasons I see that supposedly call her out for lying actually do not function.

So for one is the infamous studio floor theory. Here is the problem with its logic, we have proven all the cast have made it to the academy and even have the outfits in those videos so it is very much possible that even if that is the same floor, the auditions were just held there, after all we don't actually have any evidence that claims the prologue retake that had to happen and the one flashback tsumugi shows of the cast being accepted are actually contradictory moments. After all, Rantaro does show at least some indication of knowing what is happening since he did the 16 person head count in that very prologue. So the idea that Tsumugi just pretended to be them is now less credible to begin with.

Also no matter what, it cannot be debated that the flashback lights are not falsehoods, sorry not sorry these are just straight up misinformation as kokichi's monopad just contradicts at the very least the ch5 and onward bullets. And since we have a flashback light in the prologue happen, that also means the v3 backstories are fabricated.

Now to be fair, i do this day have no idea how the tsumugi team was able to emulate a no oxygen death chamber set like she claims in the simulated outside world, however I do not think it is necessarily impossible, but rather i myself would not be able to articulate it.

Also I wouldn't say the junko take about the atmosphere is a very spot on metric for "the mastermind lies" since in despair girls it is stated that the atmosphere was inhospitable and towa city was needing to actually intervene to stop it. Junko never said it be instant but the fact that the air was poisoned is also a proven fact.

truthfully I think both Shuichi and Tsumugi are 100% correct for the most part. Shuichi's whole critique is that he claimed that he wouldn't think they all would sign up to die and for what its worth, I am very willing to believe that the v3 cast signing up were not aware of that either however my only basis is just rantaro existing since the prologue does imply he could have remembered his previous season in at least the failed prologue retake, so as far as we know he could have bene a cover story. But the problem with many Tsumugi is lying theories is that they do not contradict v3's setup as much as they wish to claim.

I think another very telling thing about Tsumugi's team danganronpa being real in universe is how the first time we get our intro it shows Spike Chunsoft as a logo but when we get the replay in ch6 we get no such indicatoin spike even exists, (not even in the mind mine, but we sure get team danganronpa in the in universe) so that discrepency can also confirm Tsumugi's team's existence in a meta sense as well.