This seems somewhat easier from where I left off, you know. Bizarre.
Anyway, completed two zones – the Sandopolis replacement, and the Carnival Night replacement, and defeated both of the bosses at the end of them.
It really is very fast. Very, very fast.
I also managed to get into the Special Stage a few times, and grab two Chaos Emeralds. Yes, I went in more than twice, but you can only get one Emerald per Zone, it seems, so I duplicated some of them.