Plume

lmao the thing is that the first phase of the final boss will detonate one of your items for 99 damage instead of attacking if you've got any items left, so this change actually made this part of the fight EASIER

I don't believe that's currently a known bug, but uh, if you can ever replicate that, please do let me know and I'll take a look right away!

You are indeed correct on both counts—this chapter continues the story after the end of the game, so you can only start it from an endgame save. (Although you don't need to have DLC1 completed.) And yes, it does have a new progression system exclusive to this chapter, the Adversarial Zodiac! I wanted to recontextualize the party members and have a sense of progression across the chapter, but not mess up endgame builds too much, hence the new system!

Ha ha, this was definitely an instance of it being driven by vibes-based decision-making. I put in six in the initial design sketch for the new menu just because it fit the first visual layout I tried out... and it passed an initial gut check by virtue of being the same as the Lightning Returns inventory size.

I did some playtests of a single chapter just to make sure it felt right, of course, using some underpowered builds that forced me to use items relatively frequently, and it seemed to apply just enough pressure to feel right. Catching myself in a boss fight realizing that I needed to change my item layout if I wanted to win, then doing it successfully on the second try, felt like good proof that it had created the situation I was hoping for. If it hadn't worked out on that test, I probably would have had to go back to the drawing board with the layout, I wouldn't have committed hard, of course! But ultimately I think the actual number could have been anything in a pretty wide range, because the ultimate impact is just the feeling of pressure, rather than any particularly finely balanced economy.

I made this joke on Twitter a while back but it's also basically true: "Game design is typing 120 points as an arbitrary placeholder value that 'feels right', then finally months later actually breaking down the average rate, picking a firm range for how many fights you want the player to do, doing all the math... and replacing it with 130 instead." It's important to test, but honestly, a lot of the time the initial gut feeling turns out to be correct!