Once again, there are no reasons to limit this question to Special Relativity.
The fact is that a clock on board on Mercury really is ticking slower as one at Earth.
Hence an observer (a clock) on Mercury must have "lost" 6 years due to GENERAL relativity, (compared to a clock at the Earth) after one MW orbit of the solar system.
This give you 2 simple possibilities
An observer on Mercury must have seen the Sun (orbit the MW) faster as an observer at the Earth have observed the same event (faster as the 250 km/s - the speed we on Earth observe the Solar system (the Sun) is orbiting the MW) - I don't believe you can sell that to anyone because that would in the end violate that “c” always is the same.
That distance is not the same for both observers. - This is logic math since an observer on Mercury would see the Sun complete 1 orbit of the MW in less time (6 years less) as seen from the Earth.