Update: Now I have been able to convert currencies in the Sandbox successfully. So my main problem, being able to test the reading of mutations, has been solved. I think every developer would appreciate being able to interact with the Currency Cloud accounts through the API a bit more, though ;)
For whatever reason, sometimes the conversion doesn't happen immediately in Sandbox. Not sure if that's supposed to emulate how it might work in Production, but it makes testing a bit more tedious.