It is legally incontrovertible that the 16th Amendment was properly ratified.
This matter was the subject of a court case, United States v. Thomas, eventually heard before the Seventh Circuit Court of Appeals. It found against Benson, and for the validity of the Sixteenth amendment:
Thomas insists that because the states did not approve exactly the same text, the amendment did not go into effect. Secretary Knox considered this argument. The Solicitor of the Department of State drew up a list of the errors in the instruments and — taking into account both the triviality of the deviations and the treatment of earlier amendments that had experienced more substantial problems — advised the Secretary that he was authorized to declare the amendment adopted. The Secretary did so.
Although Thomas urges us to take the view of several state courts that only agreement on the literal text may make a legal document effective, the Supreme Court follows the "enrolled bill rule." If a legislative document is authenticated in regular form by the appropriate officials, the court treats that document as properly adopted. Field v. Clark, 143 U.S. 649, 36 L.Ed. 294, 12 S.Ct. 495 (1892). The principle is equally applicable to constitutional amendments. See Leser v. Garnett, 258 U.S. 130, 66 L.Ed. 505, 42 S.Ct. 217 (1922), which treats as conclusive the declaration of the Secretary of State that the nineteenth amendment had been adopted. In United States v. Foster, 789 F.2d. 457, 462-463, n.6 (7th Cir. 1986), we relied on Leser, as well as the inconsequential nature of the objections in the face of the 73-year acceptance of the effectiveness of the sixteenth amendment, to reject a claim similar to Thomas's. See also Coleman v. Miller, 307 U.S. 433, 83 L. Ed. 1385, 59 S. Ct. 972 (1939) (questions about ratification of amendments may be nonjusticiable). Secretary Knox declared that enough states had ratified the sixteenth amendment. The Secretary's decision is not transparently defective. We need not decide when, if ever, such a decision may be reviewed in order to know that Secretary Knox's decision is now beyond review.
A point-by-point rebuttal would be (ahem) pointless. The view of the courts was that the declaration by the states that they had ratified the amendment, and the assessement by the Secretary of State that sufficient states had done so, is enough to validate the amendment. Wording variations were considered too trivial to affect the outcome. Even if Benson's historical facts were 100% accurate, they would not affect the validity of the amendment.
The promulgation of these arguments has in fact been declared to be fraud.
"The argument that the Sixteenth Amendment was not ratified and variations of this argument have been officially identified as legally frivolous federal tax return positions for purposes of the $5,000 frivolous tax return penalty imposed under Internal Revenue Code section 6702(a)"
See also this dismissal from the IRS.