You are missing a very important point here Paulbee, Urahara's involvement....
Question: What are those mouth's reminiscent of?
Answer: As Zero already pointed out. They resemble the accelerated corosion of chains that remain after severing a soul from it's body.
Question: What was the method for forcing/accelerating the corrosion of the chains?
Answer: The huge dark, pit.
Question: What happened to Ichigo and Renji when they started Oetsu's "training"?
Answer: They fell into a huge dark pit
The explanation here is simple, Urahara copied and/or modified Oetsu's method of creating Asauchi. The purpouse of the pit was not, to bring out the shinigami powers. It was to "create" Zangetsu, by creating an Asauchi. Urahara "gave" ichigo an Asauchi, and Ichigo fused with it. So he doesn't have a sealed - regular katana - version of the zanpaktou, and thus, has to go Shikai-mode to materialize the sword in the first place.
It's because he was fused with his Asauchi that Zangetsu now cannot be stolen.
And he fused with the sword because Urahara actualy improved Oetsu's original method.
The only person that [arguably] doesn't fit is Zaraki. The rest (Renji, Rukia, Gin...) Were never shown to possess a zanpaktou while still in regular clothes (i.e. non-shinigami). But Zaraki, when first introduced, already had the scar, and even talked about Yachiru, which implies that he was already very powerful at the time. And probably was a shinigami for a long time.... maybe even a villian, but then he lost his memory. And forgot he was a shinigami and started terrorising rukongai. :D