I have heard those arguments before. After years of denying any culpability, the Rosenberg apologists had to concede they were spies after the release of the Verona papers wherein Soviet documentation did confirm their status as spies. They moved to a new tack that they were innocents aiding an American ally and as a backup the information was not really that important.
1. The Rosenbergs and their cabal went into business before 1941 when the Soviets had a pact with the Nazis. They were communist Soviet partisans and the fact that the Soviets later became a US “ally” during the war was only coincidental to their mission as Stalinist supporters who wanted to advance the Stalinist cause even to the detriment of their country
2.The US offered Marshall aid assistance to Stalin. We aid Palestineans and we also supplied aid to Iraq during the Iraq Iran war. Does that mean anyone supplying nuclear secrets to Hamas or Saddam during the Iran Iraq war should not be treated harshly?
3.The Soviets whenever they mention that the Rosenbergs did not supply them info that led to their development of the bomb are disingenuous. The Rosenbergs supplied them with the corroborative drawings to corroborate info they already had from and Hall and Fuchs who were part of the cabal of spies. That is if we are to believe the Soviets at all.
4.Pope Pius advocated vs their being put to death. Irrelevant. The Church has opposed the death penalty for decades. Popes are always sending entreaties to death penalty state governors to commute death penalty sentences.We should pay them no heed.
Spies for allies do time and/or expatriation. They never get executed. The Rosenbergs were executed.
A pact with the Nazis in 1941 did NOT make the US an enemy of the USSR, we didn’t go to war with the Nazis ourselves until after the USSR did.
When did Popes start objecting to the death penalty? That really started only recently, with the papacy of Pope John Paul II in 1978. Before then, historically, the Catholic Church supported the death penalty in appropriate cases.