More on Mud
While it flows, it tends to stay mixed. When it stops, as you point out, it will settle and the fluids rise. This is where all those little cones come from - they are escape vents for the CO2-rich fluids.
As for the northern plains and the channels, remember we have a 2-stage process.
First, 1-2 km of mud is emplaced, levelling the basins. Then later the channels flow in. When they hit the flat plains they spread out into thin sheet deposits because they're very fluid and mobile. This happens with the analogues on Earth, too.