You see…on the one hand if one looks at the older mortal theology sources…one find it said directly that a circumstance can change an act into an evil one. And they sometimes give examples.
For instance both Jones…and Prummer OP say this.
But then one has this quote that you provided in the Catechism:
“Circumstances of themselves cannot change the moral quality of acts themselves”
and so one has to ask --what does this mean?
Does it mean that one needs a evil intention too? as you assert.
(not to say that intention is primary for understanding an act …for object is…but that is not what we are talking about here)
and thus theology has progressed in understanding here
or does it mean something else?
For instance the phrase
moral quality of acts themselves -which may refer to such that “prayer” for instance is still objectively good…(this is speculation here) --or it could mean something else.
The Catechism and VS focus’s in on the fact that a circumstance can not change an evil into a good. and there is no explicit mention that a circumstance cannot change a good act into an evil…
The Catechism states too that:
1755 A morally good act requires the goodness of the object, of the end, and of the circumstances
together.
which infers at least that an act can be evil cause just one part …in this case circumstance is bad…
and one finds this expressed by others as that one needs a good object and good intention and good circumstances…which is true of course…and that if any one is missing the act is evil.
Perhaps though – you may be correct insofar as an evil intention or evil object are always involved in making an act evil (at least so far as guilt is concerned) or perhaps the fact that some circumstances are objects themselves…
again just reasoning this out…
in all thinks…let us look to the Church! but sometimes we have to work a bit to understand