The fact that it is classified as a comet that entails it is a small piece of rock. Given the effort the astronomers put into classifying whether pluto is a planet or not, it is unlikely that the comet is not in fact a comet. It is even more unlikely that it is a brown dwarf since a brown dwarf has a mass that is at least 10 times the mass of the jupiter, and have the same size as jupiter.
In addition, I think the magnitude of the force is irrelevant since what matters to an earthquake is that the forces acting each side of the tectonic plate is different. At that distance the difference will be even more minute. That's the same reason why 'supermoon' cannot possibly be an explanation either. The change in the gravity is already minute, and the change in the difference between the two plates will be even more so.
To further put the numbers 400N to 40,000N into perspective, consider the amount of forces involved in a fault:
according to http://www.whoi.edu/science/GG/people/jlin/papers/Freed&Lin-Geology-2002.pdf
the pressure buildup over the San Andreas fault over a 10 year period is 2-3 bars, which is 200,000 Pa, that is 200,000 N per square meter of the fault. For a fault that is 100km wide and 10km deep, that's about 10^14 N.