It may be some time before this can be proven, but they must reconsider some assumptions as they must with every exception to the rule. They believe it has an iron core because it has magnetic field. Perhaps they can send vibrations through somehow and study the echo in the same way they use a seismic echo from an earthquake to learn more about the earth's core. This idea,would no doubt have been considered, along with others. The question may be, how close to the surface of mercury the source of the vibration should be, the nature of the wave, and whether it can be done economically. Of course before then a completely different method may be discovered that will give an accurate result, but is as yet locked in the future, and beyond anything we can imagine at the moment.