Caveats in reverse order: Option (4) would be the conventional dismount and is probably the safest course. But it is fraught with risk because if her horse doesn't win, and stripped of Presidential immunity, Gloria Arroyo could face years of prosecution.
Option 3 is interesting for its novelty and would be legal. She could run with Noli de Castro for example, win the Vice Presidency and have Noli resign for her to have six more years. (Yippee!)
Option 2, as Panganiban points out, would be highly unpopular and even face international censure.
Option 1 -- the martial law option -- is the most important and fascinating. Long before either the writ of habeas corpus is suspended or martial law imposed, Filipinos ought to familiarize themselves with the following provision of the Constitution. Although the 1987 charter is reputed to be full of anti martial law features, the reality is, under the present provision, the Lower House, by itself, in conspiracy with the President, COULD impose martial law indefinitely.
However, I disagree with Art Panganiban that under a legislative-executive martial law regime, that they could then convene Con-Ass and switch to a unicameral Parliamentary system. The provision itself is clear--"A state of martial law does not suspend the operation of the Constitution, nor supplant the functioning of the civil courts or legislative assemblies..."
1987 Article 7 Section 18. The President shall be the Commander-in-Chief of all armed forces of the Philippines and whenever it becomes necessary, he may call out such armed forces to prevent or suppress lawless violence, invasion or rebellion. In case of invasion or rebellion, when the public safety requires it, he may, for a period not exceeding sixty days, suspend the privilege of the writ of habeas corpus or place the Philippines or any part thereof under martial law. Within forty-eight hours from the proclamation of martial law or the suspension of the privilege of the writ of habeas corpus, the President shall submit a report in person or in writing to the Congress. The Congress, voting jointly, by a vote of at least a majority of all its Members in regular or special session, may revoke such proclamation or suspension, which revocation shall not be set aside by the President. Upon the initiative of the President, the Congress may, in the same manner, extend such proclamation or suspension for a period to be determined by the Congress, if the invasion or rebellion shall persist and public safety requires it.Many people, including this writer, have become familiar with several key provisions of the 1987 Constitution as a result of President Arroyo's expert assaults and skillful utilization of them. This bitter experience continues and if anything promises to get worse as Gloria Macapagal Arroyo refuses to dismount the Tiger. I suppose that before it actually happens, people really ought to anticipate her next dire move and come to understand the provision that will surely come into play if GMA plays the Martial Law Card.
The Congress, if not in session, shall, within twenty-four hours following such proclamation or suspension, convene in accordance with its rules without need of a call.
The Supreme Court may review, in an appropriate proceeding filed by any citizen, the sufficiency of the factual basis of the proclamation of martial law or the suspension of the privilege of the writ of habeas corpus or the extension thereof, and must promulgate its decision thereon within thirty days from its filing.
A state of martial law does not suspend the operation of the Constitution, nor supplant the functioning of the civil courts or legislative assemblies, nor authorize the conferment of jurisdiction on military courts and agencies over civilians where civil courts are able to function, nor automatically suspend the privilege of the writ of habeas corpus.
The suspension of the privilege of the writ of habeas corpus shall apply only to persons judicially charged for rebellion or offenses inherent in, or directly connected with, invasion.
During the suspension of the privilege of the writ of habeas corpus, any person thus arrested or detained shall be judicially charged within three days, otherwise he shall be released.
Just perusing this provision will reveal much terra incognita that ought to be threshed out now. For example, WHERE is the vote to be taken to either reject or uphold a declaration of martial law or suspension of the writ of habeas corpus? The Batasan in Quezon City or the Senate in Pasay City? Who convenes and/or presides over such a session?