"If we only had someone who saw a [fantasy conspirator] conspicuously coming into the building carrying a large HEAVY paper package." The bag was found in the building. Thus, we know that it was possible for someone to have carried it in unnoticed. That includes Oswald. And there is a witness that confirms that Oswald carried a long package into the building that morning that was not his lunch and for which there is no other accounting for except as the bag found with his prints on it on the 6th floor.
Frazier saw him carry it into the building. His sister saw Oswald carrying a large package with him as he walked to Frazier's car. I know I'm a lone assassin sheeple who believes "thuh government" but I'll wager that he brought it with him to the building. He didn't leave it in the car.
You're responding to a person who doesn't believe Oswald went to Mexico City. All of the evidence that he did was faked, planted, manufactured to make it look like he did.
And I'll go back to asking the type of questions that the conspiracy believers hate: "If Marina and Ruth Paine and others were coerced into giving false testimony then why didn't they coerce them into saying they saw Oswald with a large package/bag that morning?"
They can't answer that. It causes all kinds of angry responses. Why? Because if they are forced to answer them, if they are forced to consider it, then their entire "Oswald was framed" theory is severely challenged. A kind of conspiracy cognitive dissonance ensues and they don't know how to react other than to be upset.