Tax law is extremely convoluted. Anyone who is doing anything other than the short form has probably broken IRS rules or laws at one time or another and usually not on purpose. Trump doesn't do his own taxes. He'd be foolish to do his own taxes. So if we get his tax returns we are getting what his accountant did, not what Trump did. Granted, no matter who does our taxes we are responsible, but the idea that we will find that Trump himself did some nefarious thing in filing his taxes is kinda dumb. Perhaps his accountant is dirty?
I doubt that there is more than one senator or congressman that would understand Trump's tax returns if they had access to them.
I doubt that his tax accountant is dirty.
A quick internet search, including The Security and Exchange Commission's website, shows that one of Trump's Companies paid Ernst and Young over half a million dollars for tax services several years ago. Ernst and Young is one of the Big Four international accounting firms.
I assume that one of the Big Four firms are preparing his tax returns, if not Ernst and Young. If not one of the big four, then I assume a second tier firm is preparing the returns.
Treasury Circular No. 230, section 10.34 spells out some of the standards that tax return preparers must adhere to, not to mention professional and ethical standards.
However, it is highly likely that his returns may include one or more aggressive (but not abusive) tax positions.
Even if his returns had no aggressive tax positions, making his returns still subject to audit available for the scrutiny of his enemies would be foolish, to put it politely.