A little too much different stuff in there, honestly. Also if he really was exiled by the other Makuta there's no way they would have allowed him gaining so much influence.
So some thoughts:
In theory possible, but not until after the Great Cataclysm. So this is a rather bad choice for a "starting point". However, I like the idea of him being intrigued by the concept of reproduction linked to this discovery.
Taking this into consideration to what I just said about Aqua Magna Rahi - at this point Teridax would definitely rather have executed your Makuta than exiling him. In general exiling someone who isn't already in hiding isn't something the Makuta usually do.
Like I said, the other Makuta wouldn't let that happen.
Rahkshi armor is made from Kraata. Kraata are made from Antidermis. Makuta are Antidermis. No one except Makuta themselves can create Rahkshi.
Sounds liek a bad deal for the Shadowed One.
Interesting idea, though - if your Makuta had been exiled from the Brotherhood, what about the Shadowed One granting him exile? I mean, having his own "pet" Makuta definitely might be something teh Shadowed One would like. Of course this would have to stay secret to everyone else.
I'd rather leave that out.
Check the Kanohi list (linked in the rules in teh first post) and see what's still available on there?
Not necessarily. Until recently I assumed it was Tridax, but apparently the Order of Mata Nui possessed the Olmak before Tridax got his hands on it. I'd rather not mix the Olmak into this backstory here, though.
Rules and list in post 1
All Makuta are currently assigned