If one files an "enlightened" tax return containing rebuttals to false claims that one's earnings are federally-connected, and one's return is not accepted by the IRS, what is their most common response?

If you honor their request and send them the original W-2's and 1099's that you received you are negating your own testimony on the substitute forms, which they can then ignore.

To be sure, they can send other responses, such as a threat of penalties for filing a "frivolous" return, or other consequences not actually permitted to them. Many cases in which the IRS eventually accepts an "enlightened" return begin this way, with some sort of bluff, but it is not uncommon for them to first declare that some documents are "missing" and ask that the filer provide them.

"In a recent conversation with an official at the Internal Revenue Service, I was amazed when he told me that 'If the taxpayers of this country ever discover that the IRS operates on 90% bluff the entire system will collapse'". -- Henry Bellmon, Senator (1969)

"The Tax Code represents the genius of legal fiction... The IRS has never really known why people pay the income tax... The IRS encourages voluntary compliance, through FEAR." -- Jack Warren Wade Jr., former IRS officer in charge of the IRS Nationwide Revenue Officer Training Program, in his book ‘When You Owe The IRS’

Many filers of "enlightened" returns eventually accepted by the IRS engaged in an exchange of letters in which they firmly declared what they knew to be the truth about the income tax laws. The IRS responded with different gambits, many with clever language that sounds more threatening than it actually is. Ultimately, in thousands of cases, the IRS eventually acknowledged the truth and issued a refund check or dropped a claim for back taxes against the filer.