What precisely was stopping the US from telling the USSR to back down before the US decided that if it's lost anyways, it might as well drag the USSR down with it? And then launching a full first strike. I mean apparently the laser can fire every thirty seconds, and lets assume that every shot has perfect accuracy. That knocks down about 30-34 ICBMS before the next dozen plaster the entire operating site. (1) Everyone loses in the end, and all because you didn't offer your rival a way out that would save face.
The laser wasn't for hitting incoming ICBM's, it was for hitting cities.(2) As I said, this was very much a story designed for showing what MIGHT be a scenario for forcing the USA to surrender, OR how the US government might reply to such a science-fiction level threat. Mind, at the time, the threat wasn't seen as being so "Flash Gordon" as it would today. The use of something called the FBOS, or Fractional Orbiting Ballistic System, was supposed to use nukes in orbit to nail ICBMs post-launch when at apogee. The problem with THAT idea was that they could just as easily be used for a first strike capability that gave the other side only minutes warning. It was one of the few new weapons systems that Brezhnev did NOT immediately fall in love with, but rather was ready to sign a treaty eliminating them ASAP before even SALT I. The ABM Treaty.
1) TBH, the author had the US administration so terrified of a "Soviet Reaction" that they never even considered making the threat of either a US First Strike OR a US Second Strike. Like I said, this was in an era where EVERYTHING wrong in the world was all the fault of the United States. The writing came off the pages literally shrieking of the "Vietnam Syndrome", which while bad enough in 1971 was only going to get worse when Saigon fell.
The novel's SecDef was making arguments so severely defeatist and self-flagellatory for the US in general that in any other administration in any other such crisis would have gotten him arrested. As it was by the time the SecDef demanded that the USA surrender, the author got himself out of that problem by having the SecDef promptly die (right in front of the Chairman of the JCS) of a heart attack.
The Soviet laser facility was ringed by their most up-to-date SAMs (nuke-tipped, which the Americans didn't know about) and hoards of ADF interceptors. Also KGB & GRU troops. The problem with the Soviet Plan was that they didn't allow for suicide attacks.
2) Hence, the idea for the very much science-FICTION idea of Soviet giant "reflector" satellites to overcome the curvature of the Earth, and make possible the idea of a force threatening annihilation by a bunch of "crazy bolsheviks". The US already knew that the previous Soviet General Secretary had been deposed, (3) so the POTUS and his administration feared the worst.
3) By a known hashish addict.


But he was only a civilian figurehead employed to put a non-military face before the public.
Though the author never said so in so many words, he left it apparent that the war plans set forth by the Chairman of the Joint Chiefs (4) were going to be followed if the SR-71/Special Forces strike failed. BTW, I just remembered, the SR-71 strike wasn't a decoy so much as a backup in case the strike team failed. The SR-71 turned tail only when the team's nuke went off. Oh, there were other Soviet laser sites under construction, but by the time they would be on-line so would be the Americans'.
4) His character was initially depicted as some kind of Curtis LeMay type (not Strangelove, and certainly not Jack D. Ripper). Very right wing, very much an "I told you so" throughout the story. But as things got worse and worse, he became more and more sympathetic. Even up to his becoming good friends with the new SecDef. Finally, the POTUS was following the C-of-JCS' ideas, which had been considered "insane" earlier in the story.
If the final strike failed, the reader was left with the idea that the US was, as you say
Alanith, going to hit the Tashkent laser facility with as many ICBMs as they felt were necessary to obliterate the threat. And for every shot that laser got off one more target would be hit in the USSR. Though the author's having the VPOTUS "written out of the loop" by leaving him on a pre-scheduled diplomatic visit in Manila was way too much. Vice-President Richard Nixon wouldn't have been treated this badly by Eisenhower in the event of a nuclear crisis.
AISI, you're right. Even in this novel, the Soviet Cabal turned on their leader when presented with the prospects of a nuclear exchange. The super-laser just means "increasing the heat" to the conflict. Inevitably the Soviuets either back down, or its WWIII. IIRC, the end result was that the US strike on Tashkent was considered a "trade-off" for the Soviet strike on Israel.