You are making the wrong argument to the wrong person. I am just stating the IRS position. I am not sure how you have determined on your own your argument as to who owns what. That isn't stated anywhere I can find. If you want to provide it that would be fine but other that that, it is just your opinion.
I would think the 4th amendment is pretty clear.
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
I didn't find anything supporting your position. Unreasonable would include seizure of personal papers to include invasion of personal privacy without a legislative purpose or the conviction of a criminal act.
Lets just say for a second, Congress has the right to look at anyone's tax returns just because they are in Congress. Do you think this will end with Trump? You don't think this will be used to exploit everyone who is in opposition of whomever has the gavel from here to eternity. Less we forget, Democrats were just recently bit in the ass over the same problem and for some reason has no ability to learn from the past.
Example: During the Obama years, Democrats didn't have enough votes in the Senate to get their SCOTUS picks through. They had a temper tantrum and decided to change the law from 60% to 50% because they had enough votes to accomplish that. They were told over and over, if you do this it will come back to haunt you. They did it anyway.
Because of there lack of forward thinking and (I want my way) attitude, Kavanaugh and Gorsuch were confirmed to the Supreme court, who are both huge 4th amendment supporters and would have not been confirmed if Democrats left well enough alone.
Any Federal Judge Democrats can find to uphold their position will be met with an appeal to Supreme court who now has the conservative majority. They will bring into the equation the fact that allowing seizures of Tax returns for political gain will supersede the 4th and will be a disaster in future events. I am not saying how they will rule but it will be a consideration. If I was a betting man, I would put a lot of money on how this will turn out.