Shredder: Ch'rell
 

Those questions are pretty much answered. The Shredder body came from the Utrom scientist he attacked after the crash in Japan.

Though, I think the better way to format those questions would be:

How and why did Ch`rell choose to call himself "the Shredder?"
How did he learn about the human Oroku Saki and the demon called The Shredder?

It's still bad to steal Shredder's name and armor. ;)
And pretend to be him...

Ch'rell is bad boy...:P

He chose to use an established persona from (this version of) Japanese history rather than create a new identity. Understandable since he knew that it was an identity that would create fear.

I don't think that the utrom Shredder wore the actual armor worn by the demon Shredder. But that does bring up an interesting question. What is it then that links the utrom Shredder and Karai to the demon Shredder allowing Karai to weaken the demon Shredder the way that she did? I'm sure just simply calling yourself "The Shredder" wouldn't be enough. I'm thinking that Ch'rell and Karai had the mystics put a spell on their armor, and it's that magic that links them to the demon Shredder.

His armor also changed over the centuries, and we've seen several different forms of it. I just question when he started using the "modern" Oroku Saki.

He just took someone's persona as an easy way to establish himself as having like a credible bloodline er whatever. Makes sense

Oroku Saki
 

But wouldn't people wonder why Oroku Saki yet didn't die?

Maybe it worked for a few centuries since people used to be very superstitious, and would think he was God or deity or demon or some supernatural being?

Maybe he adopted children similar to Karai, raised them to be his successors, before taking over again and repeating the cycle?

Or maybe he just faked his "deaths" and had another new body ready to take over as a new "successor?

It's probably best not to think about it...

I think utrom shredder drew power from the original shredder, Yoshi inferred as much when talking to the ninja Tribunal, and it would explain why Karai shared a link with the Tengu; all of the Shredders are vessels/hosts for his power in a way.

Yes, all Shredders are linked. Even though he was an imposter, he still controlled the Foot Mystics.

I don't think Tengu and Cyber Shredder were originally planned at all.