No one knows what is legal in a situation like this, because it is unprecedented. I think it is likely the courts will rule Congress has no authority to compel Executive Branch personnel to turn over any personal information provided to an Executive agency like the IRS under promise of confidentiality. They may hold that provision of the law unconstitutional or they may toss it out on technical grounds that it is effectively an ex post facto action. We will see what happens, but I don’t see how that case, which might take years to resolve, gets us to a Constitutional crisis. Further there is no legal requirement for public disclosure of tax returns. It’s not in the Constitution and not in the election law.
You also made an inaccurate statement:
All the previously elected (corrupt) presidents submitted their tax returns at the request of Congress (many times under Republican control).
Recent Presidents have made their returns public during their campaigns as it seemed politically wise. It was not done at the request of Congress.
Finally your proposed general search of Trump’s affairs to see if there might be something criminal is the essence of a witch hunt and a total violation of the 4th and 5 th Amendments.
The whole attitude you have of abusing the legal system to wage political battles is decidedly un-American. The only legitimate way to get him out of office is to get 270 electoral votes in 2020.