The freezing out problem for CO2 is hard to get around, but greenhouse warming from CO2, and especially methane might have done it. My guess is that there was abundant gases early on that are hardly present now, such as Nitrogen, and Oxygen. Someone (forgot who) proposed that CO2 would be stable, even if an ocean was present, so long as there was sufficient acidity to prevent carbonate precipitation. It was not a very low pH to meet that requirement. I have been formulating an idea that it was oxygen that collapsed very early out of Mars atmosphere, and ended up ultimately as Sulfate minerals. Something that was strongly reducing such as H2S or SO2 could do this. The pure iron that has just been claimed to be the major component of the dust could also be a (past) contributor to oxygen scavenging. I was just looking for a way out of the olivine paradox and still have modest amounts of water currently. As you mention, and I think too, a simpler answer is that all the water has been pretty much frozen and subliming since these olivines were exposed by millenia of wind erosion. That does not preclude carving of channels by surface water after these basalts were erupted, as they could remain buried and dry.

At any rate, Mars Sulfur budget is going way up, and it seems in my simple way of thinking, that Mars is kind of like Earth crossed with Io. The many thousands of feet of nicely layered sediments on Mars just scream oceans to me. The dry dust proponents have not explained how these exclusively aeolian sediments could petrify and form uniform layers so thick that now reside in highlands.

Meteorite impacts into layers of Sulfate or Sulfides would cook those minerals and release the thermal degradation product, SO2 gas. This would strongly draw H2O to form Sulfurous acid. It could, however, at temperature, also combine with O2 to form SO3. This is done commercially with V2O5 catalyst at 450 C.