My grandfather was somewhat of a wheeler-dealer. He managed to make and lose a fortune. But he passed on to my father a number of old book sets, this being just one. As for Neanderthals, Java Man, et al, I'll just refer you to the website below on hoaxes:
Homeschool World: Practical Homeschooling Articles: Hominids & Hoaxes
From what I've read, you are quite correct when you say that Piltdown Man was controversial, even in H. G. Wells time. Heidelberg Man and Cro Magnons were not hoaxes, as far as I can tell. I do not know how YEC deals with them. Being an Old Earth Creationist and Gap Theory believer, I'm not especially troubled by them.
One of the other book sets we had when I was growing up was an old encyclopedia set that had dinosaurs and predicted what space travel would be like. They showed an artist conception of a space liner, which looked a lot like an ocean liner in space, with people walking on the open deck!
By the way, I want to say thank you for the link. The Book (Volume 1) looks different though. But it's been a long time since I've seen it.