we might take the probability space in (1) to exclude an invasion of Poland, and the space in (2) to include one
That seems like an unjustified interpretation, since, according to the OP:
Two different experimental groups were respectively asked to rate the probability of two different statements, each group seeing only one statement:
“A complete suspension of diplomatic relations between the USA and the Soviet Union, sometime in 1983.”
“A Russian invasion of Poland, and a complete suspension of diplomatic relations between the USA and the Soviet Union, sometime in 1983.”
Since the subjects receiving statement 1 do not even see statement 2, they would have no reason to exclude the possibility of an invasion of Poland from statement 1.
That seems like an unjustified interpretation, since, according to the OP:
Since the subjects receiving statement 1 do not even see statement 2, they would have no reason to exclude the possibility of an invasion of Poland from statement 1.