It could be higher than that. Robin Canup and others have done simulations of the sort of collision needed to create our moon and it's a fairly specific range of speeds and angles. Even when that's correct, about a third of their simulations produced an unstable two moon system that broke down after about a century.
Further, you are right about keeping the core liquid but maybe not for the right reason. The moon has no iron core. Most likely the iron core of the incoming object merged with earth while the lighter stuff got blasted into orbit to form the moon. This left earth with a bigger than normal core than mars or venus and this made it slower to crystallize.
That's actually a large part of what I had in mind. I think the collision did add heat but I think the bigger contribution was, as you point out, the addition of iron and heavy (possibly radioactive) elements to the Earth's core that are necessary to drive the magnetic field. Of course it's also possible that the off-center collision that Canup and others speciulate about set the whole thing spinning faster, too. If the poles are about to reverse (or the magnetic field is about to disappear, as some real alarmists speculate), we'll get a good lesson in just how important our magnetic field is to life on Earth the hard way.
Past Magnetic field reversal does not appear to have bad effects on life. I suspect that most animals adapted by taking a siesta from 10 am to 2 pm and underwater animals had no problem.
A couple proponents of the artificial moon idea were on Coast last night. For some reason which I do not know, Hoagland showed up to hog their time, but they did make their point, which is that the chances of such an extreme moon are so small that the superior and more likely idea is that the moon was built by time travellers from our near future about 600 million years ago to make earth habitable.