So who was Paul admonishing in the letter to the Corinthians?
Your answer doesn't fly because it is impossible to assemble any group of people without both tares and wheat, and large groups do come together in unity all the time. The disunity is not a matter of tares amongst wheat, but exactly as it was with the Corinthians: People forming sects and divisions based on what they wanted to believe and who they want to follow.
So either Christ prayed for something to happen that hasn't (wouldn't be the only instance) or the true physical manifestation of the spiritual church is united.
You are hung up on looking at the carnal/physical *temporal* church as if it is composed only of the elect. It isn't.
The catholic (universal/invisible) church is the only one that is composed of the elect (both alive today and dead).
Christ lives in the regenerate --- only the regenerate are HIS BODY of which HE IS THE HEAD.
He does NOT live in the unregenerate. They are NOT His body, even though they sit in church pews every week.
When He returns the second time, He will separate out of the temporal church all the pretenders from the real mccoy; all the tares from the wheat; all the sheep from the goats.