Let's suppose -- for I am no expert on the history, nor am I well placed to evaluate your expertise -- that you're right, and that indeed the US in the early 1950s was stuffed with communist infiltrators and communism-sympathizers.

I don't think there is much dispute on the large scale of communist infiltration at the time, though obviously it isn't often mentioned or emphasised. One can however make a good case that what is by some interpreted as communist sympathy wasn't really such. One say easily use the same standards that are often used when declaring some historical figure had Fascist connections or sympathies, to go on and prove that the US at the time was a Communist country in the sense of being run by Communists. :) I think such a standard is pretty silly one though, both for fascism and communism.

Would our hypothetical history-reviewing rationalist, then, also conclude that communist infiltration -- even on the grand scale you say it achieved in McCarthy's time -- was not such a bad thing?

Sure why not. The US of the 1950s is a shining gem of what well meaning technocrats can do for the middle class. One can either credit them for it, or say it would have been even better without them, that is open to debate. But hindsight bias is at play here I think. The Cold War period could easily have ended in a horrible way, including the end of the modern civilization. We where very lucky.

If you looked at Stalin's USSR in the 1950s, knew about the Gulags, the famines of the 1930s, the atrocities of the Russian Civil war, the mass graves of Eastern Europe and the aggressive foreign policy (remember Finland and how they basically divided up Poland with Hitler?) now freshly armed with nuclear weapons (developed with the significant aid of spies in the US leaking the tech!)...

Isn't fearing the potentially catastrophic outcome of Communist sympathy and infiltration a really understandable position to hold?