Okay so I read all of [2] -- it doesn't support your argument like you want it to.

The author argues for impoundment pertaining to exclusive powers, which are very few and limited in the Constitution. I actually agree with the argument laid out in the piece, but I don't think it supports the idea that Trump is within his legal right to impound Ukraine aid.

The core argument is that Congress can't legislate the President's exclusive powers away, and I agree with that. The author then applies a "gloss" test which is used by legal scholars to help interpret the Constitution by taking into account a variety of historical and contemporary contexts.

They apply this test to the idea that POTUS is within his right to impound funds that intersect with this exclusive power as CIC. And they show historically it has been the case that POTUS has impounded WPC funds - military weapons, personnel, and construction.

Key arguments here are that Jefferson did it in the past, and the President is better situated as the head of the executive branch to identify waste in the huge defense complex than Congress, so he should be able to say "Actually we have enough, thanks." despite Congress wanting to spend more.

So we can agree that POTUS has a narrow impoundment power within his exclusive executive authority. The operative word there is exclusive, because where this argument fails is that we're not talking about WPC funding, so Ukraine aid is out of scope for this argument.

However, they do provide this argument. Some choice quotes from the piece you linked:

  Curtiss-Wright did not hold that the President is free from Congress’ lawmaking power in the field of international relations . . . . In a world that is ever more compressed and interdependent, it is essential [that] the congressional role in foreign affairs be understood and respected. For it is Congress that makes laws, and in countless ways its laws will and should shape the Nation’s course. The Executive is not free from the ordinary controls and checks of Congress merely because foreign affairs are at issue . . . . It is not for the President alone to determine the whole content of the Nation’s foreign policy.155 

  The dissenters, Chief Justice John Roberts, Justice Samuel Alito, and Justice Antonin Scalia, did not agree that the president’s recognition power was exclusive, arguing instead that it was a shared power with Congress.156 Like the majority, they also repudiated Curtiss-Wright. 157 Only Justice Clarence Thomas clung to a Curtiss Wright conception of presidential power in foreign policy.158 Therefore, eight Justices agreed that Congress has substantial legislative authority in the foreign policy sphere.

  In the words of Justice Kennedy, “[W]hether the realm is foreign or domestic, it is still the Legislative Branch, not the Executive Branch, that makes the law.”

With Ukraine aid, we're not talking about WPC impoundment, we are talking about foreign aid impoundment, and a very recent Supreme Court overwhelmingly agreed that can't be done unilaterally by the President, because it's not within his exclusive powers.