the low density of Deimos and Phobos is now generally attributed to their being loose rubble piles accreted from debris blasted into orbit by large impacts on martian surface. their low gravity did not allow the accreting fragments to compact so they contain large internal voids.My main evidence for water on Phobos is the moon's low density: 1.8 g/m^3, while water is 1 g/m^3, rock is something like 3, and iron like 8. It is possible that Phobos is just a rubble pile: a lot of empty space inside the body could explain its low density as well.
As I have said, if Phobos has no water, we can substitute another body with abundant water, like the asteroid Ceres. Even at Ceres' distance the water would be almost free: we can use some of the mined water as propellant for moving the rest of the water. And the distance to the asteroid belt would hardly matter: once the first chunk of ice arrived from Ceres, more chunks of the stuff could be delivered at a steady pace, regardless of the distance.
With a steady supply of propellant for launching finished solar panels off the Moon, the SPS construction program would be quite efficient. We really could build enough of them to save the Earth from global warming.
The volatile you refers to in outer solar system bodies are not propellants. they are at most reaction mass. the difference between propellant and reaction mass is propellant supply their own energy. reaction mass needs to have energy added to them to propels an object. Just having reaction mass doesn’t get you anywhere. In inner solar system the energy could potentially come from solar panels supply electric power to vaporize or ionize the reaction mass. solar panels don’t work very well much part midway out the main asteroid belt due to reduced solar flux beyond about 3AU, where solar flux is about 1/10 what it is on earth.
Where will you get the energy to use all that reaction mass?