The portions of the War Powers Act which I have read leads me to believe that it explicitly enumerates a situation in which power to act will be granted by resolution and that it is different from a Declaration of War. Are you of the opinion that any resolution regarding War Powers is a Declaration of War? A resolution withdrawing the authority to act would certainly not be a Declaration of War, but perhaps a Declaration of Peace.
Under the WPA, the President has the right to take actions which are "acts of war" even before Congress acts. That is what I find highly dubious. However, once Congress acts under the WPA, it is the same as if it had passed a free-standing "Declaration of War." As the Constitution requires, Congres has then spoken, and the power to carry out a war has moved from Congress to the President, who is the Commander-in-Chief.
With regard to the War on Terrorism, Congress DID speak, on 18 September 2002. There is no question of the constitutional ability of President Bush to conduct acts of war now, against terrorists and supporting nations, wherever he chooses.
Your post raises the question of what happens when Congress withdraws its authority. That is no different under the WPA than under a standard Declaration of War, as in WW II. If Congress, at any time and for any reason, withdraws its permission for the President to conduct war, that ends the US participation in that war, on the spot. Such a thing has never happened in US history, but it is possible, and that would be the constitutional result.
Hope this is useful to you.
Congressman Billybob