I think the interbreeding theory is extremely unlikely. It would require that the 2 groups co-habitate and that there be virtually no preference for mating within one’s group rather than with the other. This simply doesnt happen with primates or other territorial animals. It also requires that the offspring of such a mating are fertile which is also unlikely. It is much more likely that one group wiped out the other either through disease, aggression, or outcompeting them for resources. Even if the 2 groups were equally intelligent, a difference in the level of aggression of one culture relative to the other is enough to cause extinction of the less aggressive group.
You are probably right.
You have men willing to copulate with goats and chickens. A Neanderthal girl would not be any big deal.
A standard of conquest is that the winning tribe kills the men, and enslaves the women and children, especially the more attractive women.