Responding to this here since it's a headcanon explanation. Mephiles did what he did because he can not go to a point in time he was already at in his past, but could drop someone else off to run interference. He just needed the right sucker, which was Silver. Mephiles could not unite with Iblis as when Iblis took a physical form it 'gained' it's own mind, which prevented Mephiles becoming the mind in question. A primal, bestial mind, but a mind nontheless. He could not go back to when Iblis was released because he was already at that point in time fighting Shadow in the Dusty Desert. With Silver causing interference, either he would see Iblis released earlier, when he wasn't stuck fighting Shadow, be freed up from his fight with Shadow and can thus beeline to Iblis, or Iblis' release would be delayed enough for him to reach/cause it himself, and merge while Iblis had no mind to prevent it. If none of the above, well what's he lost beyond a bit longer looking for another sucker? I wonder if... might be an interesting prequel comic, this idea I just had.