There doesn't seem to be any problem with your theory if this area was previously a pole. The water would have come from the polar ice, and the ice under the ground would be from that pole. Some of these features may have been formed from the heat of the impacts melting the polar ice, other wise it could be meltwater on the pole at the time.

As the planet cooled the pole may have accumulated CO2 ice and so erosion could have occured in that way afterwards. With the polar wandering path there could have been water erosion at some point but when it got colder CO2 erosion could have predominated.

The dust like soil at Meridiani is consistent with a pole having been here. Dust storms over time would have had dust trapped in the polar ice and the dust would have been left behind when the pole moved southwards. There hasn't been anything so far contradicting a pole having been here, everything has been consistent with the idea.