1- Cristella Revi clearly has a genius level intellect, not just a scientific mind but in a strategic level too, so she isn't the type to be captured easily from the start.
2- I fell that Revi has the advantage on earth because of the structure the altans refugees left, she may have a connection with the empire.
3- The federation has to deal with the alliance too, so even if they really to put their hands on Revi they don't have inexhaustible resources to throw around, even more so when you remember they are a government like any other on earth where people have their own agendas.
4- To be simple about the other investigators: Kashu is almost lazy and doesn't seem to have any commitment based on professional principles; Capela is simple a maniac with zero subtlety; the priest ( forgot her name ) seems to be playing the long-con and don't really work for the federation, her bosses have their own desires.