Ive read the first link you posted before. I found it interesting that she argued that Nixon vs GSA was wrong and that former presidents shouldn't retain privilege. Still, Ive seen no law to overturn it.
Laurent Sacharoff's paper is an academic one and has no legal force. Former presidents can invoke executive privilege. Even so, executive privilege, if invoked by a former President, remains qualified and can be overcome in judicial proceedings. A former (i.e. Nixon, Obama) and a sitting President's (i.e. Obama) presidential communications can be released for in camera review and possibly for public consumption. As argued in United States v. Nixon:
"However, neither the doctrine of separation of powers, nor the need for confidentiality of high-level communications, without more, can sustain an absolute, unqualified Presidential privilege of immunity from judicial process under all circumstances. The President's need for complete candor and objectivity from advisers calls for great deference from the courts. However, when the privilege depends solely on the broad, undifferentiated claim of public interest in the confidentiality of such conversations, a confrontation with other values arises. Absent a claim of need to protect military, diplomatic, or sensitive national security secrets, we find it difficult to accept the argument that even the very important interest in confidentiality of Presidential communications is significantly diminished by production of such material for in camera inspection with all the protection that a district court will be obliged to provide... "
"...We conclude that when the ground for asserting privilege as to subpoenaed materials sought for use in a criminal trial is based only on the generalized interest in confidentiality, it cannot prevail over the fundamental demands of due process of law in the fair administration of criminal justice. The generalized assertion of privilege must yield to the demonstrated, specific need for evidence in a pending criminal trial." (United States v. Nixon -- http://bit.ly/2iMlKku )
As far as the GSE cases are concerned, in camera review has already occurred with Defendants' withheld documents that were assigned and logged with presidential communications privilege in the US Court of Federal Claims (Sweeney) and the US Court of the Appeals, DC Circuit.
As you know, Nixon lost the suit versus the GSA on all counts and failed to challenge the constitutionality of the Presidential Recordings and Materials Preservation Act (1974). The GSA retains complete control over former President Nixon presidential documents and communications. As stated in the PRMPA "...the tape recordings and other materials referred to in section 101 shall, immediately upon the date of enactment of this title, be made available, subject to any rights, defenses, or privileges which the Federal Government or any person may invoke for use in any judicial proceeding or otherwise subject to court subpoena or other legal process." https://www.archives.gov/about/laws/nara.html#2111-note - (This is a readable copy).
Sacharoff may be seen by some as a constitutionalist. Sacharoff argues that the Supreme Court's decision in Nixon v GSA (http://caselaw.findlaw.com/us-supreme-court/433/425.html) to recognize a right of former Presidents to assert executive privilege as it relates to a former president's confidential communications is against the intentions of the Founding Fathers. And so unlike the decision made in Nixon v. GSA that allows President Nixon a qualified executive privilege "subject to any rights, defenses, or privileges which the Federal Government or any person may invoke for use in any judicial proceeding or otherwise subject to court subpoena or other legal process" Sacharoff argues, academically, that the US Constitution, as intended, does not support executive privilege for former Presidents.