I think there is a lot here to support your theory of a white Mars. If at some stage these spiders formed many dunes that is an alternate explanation to that water formed them. Also there are problems with the terraces found at lower latitudes in that you have to have many layers built up and then enormous amounts of water to create the erosion, and there is little evidence of this water. If the spider action created them then that eliminates the need for water.
This theory I think has a good chance of being taken seriously as these structures are clearly being formed at the south pole in association with these spiders, so all that is needed is perhaps a climate change to make them do this at lower latitudes.