The stuff where Karona summoned mana representatives is in the middle of the Scourge novel. She is lonely and realizes she is destructive, so she goes to the top of a mountain alone with her Ixidor clones (yeah, at last check, two clones of Ixidor--formerly portals of his--were best buddies with Karona right before they killed her with Kamahl's Mirrari sword). There, she pulled up a strong rep from each color to attempt to find out her purpose for living.
Then comes a little planeswalking session (as in, interdimensional travel...NOT as in unblocked passage through praeries). She visits Mercadia (from Mercadian Masques) and it is not at all in good shape. Apparently, Phyrexian-enhanced goblin tyrants rule the plane. Then she skips over to Serra's universe and gets a severe word-lashing from an Arch-angel before hopping into the early Mirrodin. She talks with the Warden and then leaves.
So, you'd have to look around the middle of the Scourge novel to get the first peek ever of Mirrodin.
Later, when Karona is killed, she turns into Jeska (wtf!?!...but it happens that way) and then slips over to the early Mirrodin again to meet Karn, who apparently was posing as the Warden. Then, they slap each other in the rump, exchange horny looks, and are off to planeswalking. (okay, the sexual stuff wasn't in there, but they did leave together).
This is at the end of the Scourge novel. However, there is no hint at this point that the plane is Mirrodin. It is only unmistakably described as something akin to Mirrodin.
In the next novel, the Moons of Mirrodin, (which is written by a MUCH BETTER writer than the OLS-block books) the prologue retells Jeska and Karn meeting and deciding to go universe hopping. This time, Karn leaves the entire plane in the care of the Warden, who is obviously Karn's lackey. The Lackey sits and stares for centuries and then millenia, just wondering what the heck to do. (Yeah, seriously.) The makes little changes to the plane and names it Mirrodin, after the Mirrari which the Warden was apparently created from. (So the Mirrari exists yet, but it has been reforged into Memnarch). The Warden is clearly making a transition from lackey into Memnarch in this passage, and clues are given to the fact he wants to abduct other life forms to fill Mirrodin, then it ends.
And so begins chapter one of the book without anymore mention of the creation of Mirrodin by Karn or its twisting by the Mirrari/Memnarch.
Additionally, it has been thousands of years between the end of OLS-block and the beginning of MD5-block.
So when next we see Dominaria, the world may be totally different than we remember, thanks to thousands of years gone by.