One reminisces Bill Clinton's election campaign tagline: "It's the economy, stupid". In the same light, given the much hullabaloo on GMA's EO 464 over which the Senate cries foul, one may chime in: "It's not the constitutionality that's at issue, stupid..."
What at first blush is topically simple for resolution, in more defining perspective, the critical blind alley - a political cul-de-sac? - posed by EO 464 needs serious meditation. There's no cavil that both chambers of Congress, or their committees, may conduct inquiries in aid of legislation (Sec. 21, Art. VI, Phil. Constitution). Likewise, department heads on their own initiative, with the consent of the president, or upon request of either chamber, may appear and be heard... on matters pertaining to their departments (Sec. 22, Art. VI).
The first refers to oversight powers which either House, or any of its committee, may exercise "in aid of legislation" and, can require the appearance of department heads, and/or practically anybody in government, during any inquiry "in aid of legislation" without prior presidential approval. "The rights of persons appearing in or affected by such inquiries shall be respected", is its limitation.
The second is the privilege or "question hour" in plenary session of either chamber that may request/require department heads to appear during the "question hour". Its limitation: When the state security or public interest so requires and the president so states in writing, the appearance shall be conducted in executive session.
Evidently, EO 464 is constitutionally ultra vires on GMA's part by banning: 1) the appearance of cabinet members at inquiries "in aid of legislation" without her prior approval; 2) and also non-cabinet personnel including PNP/AFP and "other officials" or subalterns.
Did GMA and/or her legal advisers commit legal faux pas constituting a grievous ignorantia legis? Definitely not... EO 464 isn't just a knee-jerk defense mechanism against the Senate's rank disrespect for the Executive department. In essence, it was not NSA Gonzales as GMA's alter ego who was abused and called a "pimp" and "nanloloko", but GMA herself.
EO 464 has drawn the "Maginot Line" to protect and preserve the built-in Executive privilege and "intra Cabinet confidentiality" to shore up its aegis as co-equal branch. Its counter-force of check and balance cannot, and should not, render it nugatory by whatever legislative dissimulation or hypocrisy as "in aid of legislation", or whatever vacuous pretext.
GMA is right that after the failed House impeachment, the Senate "serial impeachment" now done by the Blue Ribbon, is just a never-ending ploy of destabilization. In fact, the divergent calls for her ouster overlap the borders of legitimate redress. Note retired Gen. Montano's ominous threat: "To avoid bloodshed, GMA must resign; if she doesn't, there will be bloodshed".
EO 464 is a jarring wake up reminder for the Senate to honor and obey, and with elementary civility, the constitutional mantra: "The rights of persons appearing in or affected by such inquiries shall be respected". "Your Honors" need not be coweringly awed by overbearing Frank Chavez who merely got a flick of the wrist for his odiously direct contempt. But, definitely, they should not act with excessively bullying and coarsely bulldozing superciliousness, as they did with the bumbling NSA Gonzales.
Overall, the political impasse is not really on such highbrow issue of EO 464's unconstitutionality. It's rather as simple and elemental as the lesson in elementary civics on good manners and right conduct even among the high and the mighty.