A lot has been discussed about the possibility of panspermia between Earth and Mars, and how compact systems such as Trappist-1 can be more prone to that.
It is easy to guess that on a much more compact system such as the Galilean moons, this has been happening more easily and often than on solar-system scales.
A proper study of Europa is dificulted by the hostile radiation environment on which it orbits, and this is the reason why Europa clipper orbits Jupiter instead of Europa. Juno, JUICE, and other missions will also follow orbits that avoid the radiation belts as much as possible. The proposed NASA's Europa lander would be several Billion dollars, for the same reason I suspect.
Callisto, on the other hand, is a different story. Thanks to the way lower radiation levels due to its location outside the radiation belts Manned missions are possible, orbiters can be developed whithin Discovery budgets, and even China is now on the early stages of its first unmanned mission to the Jupiter system which will focus on Callisto, with even discussion about adding a small lander.
Regardless of whether the mission is manned or unmanned, and besides the worthwhile Callisto-focused science and remote observations of the other moons:
Can we unequivocally identify samples coming from Europa?
Would this prospect strenghten the mission rationale to land on callisto?