As for Paper Jam, it's true they stated it was an intentional decision of AlphaDream to avoid OCs but I definitely feel like had Paper Mario not set a precedence for that, it wouldn't have been a restriction in a crossover. Like say if Paper Jam had released when there were only 3 Paper Mario games and 3 Mario & Luigi games, absolutely none of them would have a precedence of restricting to only mainline characters so the idea of combining both flavors of characters, different counterparts, etc would probably be more of a focus vs. just having the main cast and their doubles. So Miyamoto didn't make the call for Paper Jam, but his call in Sticker Star indirectly influenced it.
Tbh even at the time, I didn't find that interview answer entirely honest, both because it seemed just too on-the-nose after the massive backlash Sticker Star got, and because the answer was by one of the Nintendo supervisor on the game rather than an AD employee. But with this new, and the twitter thread I linked above mentioned Nintendo told the company to be more cost-effective after Dream Team, I really can buy the main motivation behin Paper Jam's lame crossover wasn't some noble idea to keep the plot or a sinister edict to boringfy the Mario RPGs for grandmas, but mostly AD being too strapped for cash to do anything else.