What is your interpretation of the last page of the book ("Descent")? Is it to be taken literally (guided missile hits movie theater in LA) or is it Pynchon's way of saying it matters not as to time and place (WWII Europe, 1960s America, etc.), weapons of mass destruction will always be with us . . . something else?

I think the final page of the book is more symbolic in nature, but that's not to say that in the events of the novel, that wasn't what actually happened. I believe there is a line that can be walked wherein realism and symbolism coexist.
The final action of the novel involves the launch of the hard-sought-after rocket, killing the launcher and traveling straight towards LA to cause mass destruction. What's interesting is this particular rocket was designed to be the first rocket to take man into space. This is extremely symbolic of the Cold War and the events that happened after World War 2. The idea is that, after World War 2, with the advent of nuclear technology and long range weapons, apocalyptic scenarios seemed ever present and could happen without any warning.
During the Cold War, the Space Race and the Arms Race were intertwined pursuits to create lasting dominant technological capabilities before the enemy could do so. In the novel, the missile's launch (representing the good technology, such as in the Space Race, being manipulated towards destructive ends) represents the idea that, because this technology has been introduced and because of the warring ideals in the world, the destruction has already happened—it's inevitable. It is only a matter of time, the book posits, before the capability for destruction leads to actual devastation.