The answer is no.
No atmosphere is equivalent to all molecules being able to evade to space (This happens on planets with low gravitational force). Thus also $H_2O$ molecules will evade and I guess that many molecules already evaded during the frozen state of the planet due to sublimation. So this could be the end of this answer.
But let's ...
Satellite measurements have been done and the sea floor is sinking relative to the continental crust so sea level SHOULD be falling. In some places the continental rise is quite extreme, the places were loss of ice has changed the isostatic balance of the continent.
Major problems with sunlight\energy\temperatures for the "southern Hemisphere". The Flat Earth Society offers this suggested map of a flat Earth and a rough idea of how seasons would work:
But because of the larger radius of the SH Summer circle, the sun would have to move faster to complete a circle in 24 hours during December. That would mean ...
The second part of the question is easy to answer - given that the Arctic has been ice free in relatively recent geological history, e.g. in the Mesozoic, and it is expected to be ice free in summer within a few decades as a result of global warming, positing a habitable planet where polar oceans are navigable, at least in summer seems fairly safe.
I will throw out 2 hypothesises to answer the why of the question. Not saying it is true, just saying that it may be valid hypothesises. And both hypothesises don't discount the internal properties of greenhouse gasses.
Somehow positive and negative feedback forces level each other out in the 1 bar thick atmosphere sytems on all 3 planets
There is another ...
I hope I can explain this well! I will focus my answer on the ITCZ in the Pacific Ocean. The first point is that the Coriolis force is zero at the equator and maximum at the poles (1). So, another factor comes into play "the walker circulation which is strongly tied to El Niño–Southern Oscillation (ENSO). ENSO states are 3: normal conditions, La Niña, ...
The factor of 1000 may be related to the expected units. The mass 'units' can be found in the derivation.
The full derivation of mixing ratio goes like this
since, by the ideal gas law,
then the mixing ratio is expressed as