After discussing things with my co-designer Albert Bailey, I made some modifications to Mercury. The Sunward Oceans - now plural - circle the globe just sunward of the inhabited lands, and in places deeply penetrating them. These seas are hypersaline, and their sunward shores are indefinite, depending greatly on seasonal variables. It is impossible to sail completely around the globe on them, as there are places where connections do not exist, but these areas are comparatively narrow, and all rivers empty into them. (I had to move them Darkwards, otherwise because of their bright reflections, they would be distinctly visible from Earth by 1880.)
Humans are more tolerant of heat than Martians, and one will tend to find more Martians the further one goes Darkward. Darkward of the normal 5 degree line of habitation, there are isolated areas where geothermal heat keeps areas clear of snow and ice throughout the "winter"; while there are also isolated areas Sunward of the Oceans where local conditions permit permanent habitations. There are also nomadic peoples - Human to sunward and Martian to Darkward - who subsist in seasonally available areas where permanent habitation is not possible.
Culturally, the Mercurians are wholly Martian, the Humans having been snatched up as slaves from Earth in our prehistory or early history, but who have long since gained their freedon, and are thoroughly integrated into the core Martian culture. Mercurians are organized into typical Martian-style "Houses". The so-called "Great Houses" are all headed by Martians - Great Houses are distinguished by being formed before the departure of colonists to Mercury - and the vastly more numerous "Lesser Houses" are those formed on Mercury, which may be either Martian or Human at the head. In any case, "Great" or "Lesser" does not refer to House wealth - many Lesser Houses are much richer than many Great Houses - but to House Tradition and Ancestry.The Heads of the Houses effectively rule Mercury, with an executive called the Greatest One, and called the "Doge" by earth humans, elected from within the Great Houses who judges all disputes between Houses and controls all off-world trade.
Mercurian Humans have strongly adapted to Mercury, as Martian Humans have to Mars. They are, like Martians and Martian Humans, tall and willowy, with greater chests and much higher lung capacity than Earth or Venusian Humans. They are recognizably human otherwise, and able, with difficulty, to cross-breed. Mercurian Martians, on the other hand, are much like those on Mars, as the surface gravity and thin atmosphere are very much the same on both planets.