Amnesty?

Hmm, while you posit an interesting argument, I believe it creates more of a 'slippery slope' than a foundation for your argument. Let's look at what amnesty is:

'Amnesty (from the Greek ἀμνηστία amnestia, oblivion) is defined as: "A pardon extended by the government to a group or class of persons, usually for a political offense; the act of a sovereign power officially forgiving certain classes of persons who are subject to trial but have not yet been convicted" It includes more than pardon, in as much as it obliterates all legal remembrance of the offense. The word has the same root as amnesia. Amnesty is more and more used to express 'freedom' and the time when prisoners can go free.'

Keeping in mind we have an important point here: "It includes MORE THAN A PARDON" (emphasis mine). This alone creates the legal question of whether or not article 2 even applies (especially since the article specifically stated "pardon criminals" which implies their conviction has already occurred in court. With amnesty, no such action occurs. This alone possibly negates any 'fiat' you believe the article conveys. The confederate soldiers amnesty was not exactly by fiat either:

"On May 29, 1865, President Andrew Johnson provided for amnesty and the return of property to those who would take an oath of allegiance. However, former Confederate government officials, officers with the rank of colonel and above from the Confederate army or lieutenant and above from the Confederate navy, and people owning more than $20,000 worth of property had to apply for individual pardons. Though it was difficult for ex-Confederates to ask for a pardon for something they did not believe had been wrong, thousands did ask for and receive amnesty from President Johnson." (source: http://www.wtv-zone.com/civil...

As for Nixon, there are a few that contended that Ford over reached HIS powers when he granted him amnesty as well (especially in light of the fact that Nixon still refused to admit any wrong doing, and he had not been formally convicted, what he was really given was immunity), in fact many agree it cost Ford his re-election bid. So both of you examples aren't exactly making your argument here.

The point is, Article 2 Section 2 is clearly being abused if this is going to be cited as the source of authority to grant fiat authority to give back door amnesty to illegal immigrants (granting reprieves and pardons implies 'already convicted'). Anyone with any sense (legal or otherwise) can see that this article section was not written to convey this and would not necessarily stand up to a legal challenge. It will be interesting to see if it happens though given the polarity of the issue.

Of all the Supreme Court cases cited (almost all were individual criminal cases), only one seemed to be applicable in this scenario, and again, the court opinion does not fully support this move. In Brown vs. Walker, the court opinion was stated:

*601 The act of Congress in question securing to witnesses immunity from prosecution is virtually an act of general amnesty, and belongs to a class of legislation which is not uncommon either in England, (2 Taylor on Evidence, § 1455, where a large number of similar acts are collated,) or in this country. Although the Constitution vests in the President "power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment," this power has never been held to take from congress the pass acts of general amnesty, and is ordinarily exercised only in cases individuals after conviction, although, as was said by this court in Ex parte Garland, 4 Wall. 333, 380, "it extends to every offense known to the law, and may be exercised at any time after its commission, either before legal proceedings are taken, or during their pendency, or after conviction and judgment."

So you can see the Supreme Court both placed no limits, but also indicated what the power was for and not. And clearly it is stated that it is was NOT to be used to take from congress the power to pass acts of general amnesty.

Clearly no law was broken, but it did open a window for a legal challenge due to past interpretation. Previous ones however did not feel this was the proper use of the articles powers. Again, the question is, would such improper use be justification for Judicial level intervention? I think many will disagree on the answer even if the Judicial branch took a pass.