Sorry, you are correct,
"In such a manner that the congress may direct”
That doesn’t change anything, they can still control it. Yes, they don’t have to direct anything, but Manchin says they can’t direct it at all and he’s wrong. It is within the power of congress to make this a non-issue without an amendment. Of course they should repeal the amendment to eliminate the problem if DC achieves statehood and/or pigs fly, but it’s not a requirement.