"So, say there is less salt, and less water and the water lasted for shorter times, but you would still have a similar process."
Yes. Since water cannot survive on Mars today, it's doubtfull that such a process can occur. Underground and if it's warm enough, maybe (and that would be something!).
"That they could come from further out and migrated inwards is possible, but Ceres has apparently a lot of water in terms of it's abundance relative to other material- 25% of upper crust, so superheating it's surface with any impactor should create a fair amount water vapor.
Ceres at the moment is thought to have a very thin atmosphere of water vapor:
"Ceres may be surrounded by a tenuous atmosphere containing water vapour."
http://en.wikipedia.org/wiki/Ceres_(dwarf_planet)"
Yes. Even Pluto is thought to have an atmosphere when it's orbit brings it closer to the sun. Brings comets to mind...