What if the seismology component was an optional extra? The ship orbiting the planet dropping impactors could have instruments for analyzing the plume, etc., and if you had previously landed a seismograph close enough then you could receive extra science points. The difference between just watching an impact from orbit vs. recording the seismic data might be on par with the difference between transmitting data about a sample vs. returning it to kerbin. I have no idea about coding any of this... I assume it complicates things to try to have two ships working together on a science experiment like this?