Genetically it doesn't matter if they became extinct or if they were absorbed in to our population. And it's not entirely true that sub-saharan Africans have no neanderthal DNA, last I read they had, but it was from humans later migrating back in to Africa.
No mate, not even close tbh. Genetic wise, there's more genetic differences within Africa then there is outside of Africa. We are much more closely related to anyone on earth than the Neanderthal or Denisovan.
But any potential gaps in the DNA sequence would be filled by the hosts genetic material which could hinder things. I'd think the most likely outcome would be an intermediate species.
I'm not sure what they can do with DNA but if they were to use a fallow deer as a host then the outcome would be a tad disappointing, I can't see the result being close to the size of the Irish Elk.
I'd love to see an attempt, just because they could. But, no, I don't think they should be recreating extinct creatures and reintroducing them in to the wild. We can't even look after our living flora & fauna as it is.