Shiniki does make cheaper shell cordovan because it had been used in school bags. Yes school kids in Japan used to carry shell cordovan briefcase and rucksack to school. You can read about it on Leder Ogawa's website. They now switched to higher end products with more vibrant color and shinier coating but I guess they still have not improved much on the process.
Absolutely. Like I said, it's not actually a bad leather for its intended purpose, but the hype machine around other Shinki products has caused the same product that was considered cheap and low quality 13 years ago to be considered automatically higher end.
Those are sometimes hailed as special color but are actually rejects. Not to mention the hype about rare colored shell.
Maryam is closer in terms of quality to Horween than Shiniki.
Maryam shell is definitely quite nice.
As for rejects explained as as 'special handmade process, rare color!', this happened just a few months ago with a batch of flawed Shinki horsehide (not shell) that was used in a run of Lofgren boots.