Of course, there are other sequence variations that are found in modern humans and in Neanderthals – and chimps, monkeys, mice, fruit flies, etc. I suppose one way of asking your last question would be whether all the DNA sequences (or almost all?) found in the Neanderthals we have sequenced (a handful, at most) can also be found among modern humans. If so, one could imagine (very difficult in practice, of course) “backbreeding” with modern humans to get a child born of modern humans that had an entirely Neanderthal DNA sequence. I guess I would still say the Neanderthals are “extinct” but that they could be revived without genetic engineering.

I would be very surprised if all the Neanderthal-specific sequences can be found in modern humans, but i don’t know – and I don’t know whether it is only 2 to 3 percent of Neanderthal-specific (or, probably more accurately, “Neanderthal present; modern human not (very) present”) genetic variations that have been found in (some) modern (non-African) humans or 20 to 30 percent or 80 to 90 percent. My guess is toward the lower end, but I’ll ask some experts.