You raise a lot of problems that I think we should all start considering. That is why I am so supportive of the TEDx event specifically focused on de-extinction. It will raise public awareness and get people thinking about what this means for the future of the biosphere.

For your first questions I think you are focused on the biggest issues (wildlife ecology). You can't just bring back extinct animals - you have to try and recreate ecosystems that no longer exist. Bringing back a saber-toothed cat and placing it in a contemporary environment would be extremely problematic. Experts from paleontology, ecology and conservation biology will need to organize and come up with species-specific plans if this is going to happen.

I do not support the de-extinction of Neanderthals for many reasons which I outlined in a previously post. The biggest reasons being that Neanderthals would a) be conscious beings that would struggle to find an identity in the modern world and b) our history of discrimination and conceptualization of intelligence indicate that there would be massive tensions between our species and another human species. I would also worry about the first Neanderthals being exploited by reality television and scientific studies.