Take a look at this sequence of Sol observations:
Notice how the longitude of the SRV plus the altitude of Sol is constant at between 55 and 56 degrees. I think the spin-orbit of Mercury is not modelled correctly and seems to behave as though the planet is tidally locked and always presenting the same face to the star.
Previously, I thought I had measured the terminator moving westwards at 1.1 degrees per day and made various predictions about where it would be. I think this is now wrong, in fact to paraphrase Nixon: "(EXPLETIVE DELETED), all previous statements regarding the position of the terminator are inoperative."
If you approximate the orbit of Mercury to take 90 days (88) then if Mercury didn't spin the terminator would move eastwards at 4 degrees per day.
If you approximate the rotation period of Mercury to be 60 days (58.6), then if Mercury somehow didn't orbit Sol but simply rotated then the terminator would move westwards at 6 degrees per day.
Combining these values, the terminator should be moving westwards at about 2 degrees per day. The first observation above, the one at longitude 39 was taken 2 days ago so there should be a 4 degree difference in altitude of Sol due to this spin-orbit combination. That hasn't happened.
Yesterday, I did a 6 hour experiment in which I parked the SRV looking at a shadow to see if it moved at all.
In a quarter of a day, Sol's altitude should have changed by half a degree, but the shadow hasn't lengthened by any noticeable amount.