#1 - Congress can alter out tax system via a lot of non-tax bills. In 2015, we saw ten bills enacted (as of 12/11/15) with tax changes. Yet, these bills were not intended to be tax bills, they all had a different primary purpose such as enacting trade deals or funding the Highway Trust Fund. Various tax changes were added in, many of which had been around for a while. For example, the GAO has been suggesting for years that additional information be required on Form 1098 mortgage interest statement. The change in due dates for some tax forms has also been talked about for some time and was even in Congressman Camp's H.R. 1 (113rd Congress) tax reform bill.
Relevance - this is really not the best way to improve our tax system. Isolated changes, some of them significant, such as due date changes and changes in the audit procedures for partnerships, should really be part of a deliberate bill to improve our tax system. Efforts to reduce the tax gap, such as by denying passports to some delinquent taxpayers is good, but what else is also needed? But, perhaps it is easier in Congress today to just tuck in changes that are of interest to enough members to be included or perhaps not of enough interest to others to hold up a bill. Certainly, if there is some dire need to fix something, inserting it into the next bill to be voted on may make sense, but I think our tax system would be better served by looking at the system as a whole and improving it.
Here are the ten laws enacted in 2015 (as of 12/11/15) that included tax changes: