Does this mean that pre-Earth was roughly 9 Moon masses (~10%) lighter
than today's Earth? Or could much mass have been ejected from the
Earth-Moon system?
I think that's likely about right. In a nutshell, there's two basic questions here. How much mass was lost as a result of the giant impact and how much mass was gained after the giant impact during the late heavy bombardment and following collisions.
We know that smaller bombardments on Mars has resulted in debris landing on Earth, so it seems highly likely that some debris was lost when Theia and Earth collided, but estimating how much is tricky and you'd really need to be well versed in the models to have a pretty good guess.
One theory is that Theia formed in an Earth Trojan point so it shared the same orbit around the sun that the Earth did. Source This would cause a slower impact than a more oblique approach where the velocity of impact wouldn't be much greater than the Escape Velocity of Earth. If the impact velocity is only a bit above the necessary escape velocity and if a fair bit of energy is transferred into orbital momentum and heat, it's unlikely that a high percentage of debris would reach escape.
Now if it was a faster impact from a more oblique orbital approach or a head on collision that would create greater compession and rebound, those two scenarios would likely lead to more material escaping. I can't give you precise numbers, this is more of general thinking about what might happen.
Again, according to Wikipedia about 20% of the mass of Theia entered the earth's orbit after the impact. Orbital Velocity has to be at least Escape Velocity over the square root of 2 and less than escape velocity. Source
So if we use 11 KM/s as an escape velocity estimate, anything that flew off the impact at less than 7.5 KM/s would have returned to Earth pretty quickly. A bit above 7.5 and maybe about 9 or so KM/s would be low Earth orbit and about 10 KM/s high earth, with 11 KM/s, likely escape and enter in a near earth orbit around the sun. You'd need a bit faster than that to have a truly outside of earth orbit.
What percentage of Theia flew off at those velocities is hard to say, but I think it's likely that much of the planet slowed down upon impact and not too much escaped the Earth's Hill sphere, but I'd be guessing if I tried to put percentages to that.
Did the impact substantially and immediately change Earth's orbit,
such as its average distance from the Sun?
Because it was a (I assume) a Trojan object, the 2 young planets would have shared the same orbit and so there probably wouldn't have been a big change. Orbital momentum should be largely be conserved though some orbital velocity might be converted to rotational momentum, I don't think there would be a huge change. Similarly, if any debris from the impact flew off in one particular direction, the remaining material would react with similar momentum in the opposite direction, but having much greater mass, the effect would be small.
Certainly some orbital change probably happened, but compared to the entire Earth, it was probably small.
Now, if the impact was different than theorized, that might change the answers quite a bit. (My 2 cents anyway)
