One limitation: Harbin's altitude is listedby Wikipedia as only 150 m, which is lower than for instance Chicago. The altitude difference is not really there, leaving Venice's access to the sea a more decisive factor.

Most calcium-bearing rock is compounds that don't readily dissolve in water; you have to have some acidification. So I would not suppose calcium-bearing rock (or any rock) would spontaneously render rainwater basic.

Magnesium is known to form some more volatile organic compounds and it too can make water basic, but again such compounds would not get very far before reacting in our atmosphere.

Gravity is not weak at the C-M boundary. Because the core itself is so much denser than the mantle, the gravitational acceleration does not drop off until we actually enter the core and start putting that dense material above us. The C-M boundary is actually the surface of maximum gravitational acceleration on Earth.

If there is more iron versus magnesium in Martian rock, that could influence the course of serpentinization. Magnesium displaced from the rock forms brucite, from which water could be recovered by reacting with an acid like carbon dioxide. But iron forms magnetite plus elemental hydrogen, and the hydrogen could escape making water impossible to recover.