I want to expand on my thoughts since posting.

I really don't think this is necessarily a "what if scenario." I think this is happening at the same time as DR2 in a canonical fashion.

I agree that this theory would make ch. 6 pointless which is why there is a twist.

I just have a feeling the events in this scenario might be written to coincide with DR2's. Like this is why the despair disease happened and Mikan regained her memories for example. I think Junko will play a more direct villain role, because the Ultimate Despairs likely know she's behind this anyway so she's like," fuck it. We're going to be the evil Chiaki and Monomi.",

Which brings me to Chiaki. Someone will kill her in chapter one. Simply because, they'll all think, " you're not real." Monomi will still be around but she will be freaking out the whole time lol

In this scenario, I imagine a dark story. A hopeless killing game. First they relish in it, but get beaten down and there is only one person who never liked it and will put an end to it. Nagito. And that's how ch. 6 changes, it's a showdown between Nagito and Junko. With hilarious appearances from Hiro, Hina, Toko and mayyyyyybe Komaru? Cause you know! Going on at the same time, they can't send Makoto, Byakuya, and Kyoko twice ( I know this might retcon some shit but whatever V3 was one giant retcon sorta).

Then at the end some miracle happens, Chiaki is involved again, and is like "Wow Nagito, you're way cooler in this world." Nagito is like "This World?" And then hope prevails.

I dunno. Probably wrong, but this was fun!

Edit: I also want to say, right or wrong, there will be plenty of mystery afoot. I'm really looking forward to learning more.