Again, it isn't that clean, if it was a head on impact it would have ejected far more material and the heat would have been far greater.
We can tell from simulations which have since been ran that the impact must have been from a fairly shallow angle, and it would have scooped a significant amount of material out.
Yes, molten magma does contain water in it still, if it is not able to percolate out before it cools, this is what happens in the mantle of the Earth, plates get subducted, the rocks melt, the water in the rocks gets baked out and floats upwards, pressure, etc.