Re: Water, water everywhere...
I'm not a physicist, but I'd guess it'd be hard to circularise the Earth's orbit if it had once been in an elliptical orbit and crashed into a smaller Mars-sized object. It's much easier to explain the Earth-Moon system if you let an object about 10% the mass of the Earth do all the colliding.
Now I can go back to being a geologist... it's worth remembering the impact between Theia and the proto-Earth was just the last large impact we have evidence for here on Earth. All of the models of Solar System formation predict that the very early Solar System would have had several Mars-sized bodies blundering around the Inner Solar System waiting to crash into one another.
The Earth of course would have then gone through the literal hell of the Late Heavy Bombardment (4.1-3.8 Ga) in which there would have been several impacts carving out basins in excess of 5000km. This would have been enough to reduce a good part of the Crust to rubble and caused massive, shallow melting. But nearly all the evidence has been destroyed by nearly 4 billion years of tectonism and (in Wales nearly continuous) rain.