Wouldn't it also be able to be shot down by interceptors or SAMs, a disadvantage an ICBM doesn't have?
False question.
ICBMs could be shot down in the 1960s. If you don't mind pumping the Van Allen Belts (killing every satellite up there for the next few decades) you can get very large kill radii out of nuclear armed interceptor missiles (SPARTAN's 5 MT warhead was to have a kill radius measured in hundreds of miles).
The point (to some degree) of PLUTO was that it had the power to go very fast very low. Because it was to fly so low, it couldn't use stellar navigation, which given the limitations of inertial navigation means TERCOM, which can be spoofed.
If Catz flies her PLUTOs at 30k feet, TERCOM can't be spoofed as well, but the target will get longer ranges out of their tracking radars, and the PLUTOs still have serious heat issues (even the SR-71 and the XB-70 had heat issues at mach 3 and 75k feet). Plus, as you mention Carnac, they will be vulnerable to SAM fire.