Was it actually suggested that Ultimecia can bring with her her own powers to the past when she possesses people through time? I always figured passing through stuff just was one of Edea's own powers.

Also, even if ultimecia can bring life to monsters, that doesn't mean she can bring herself back to life if she dies. She was alive when she gave life to other things. We don't have anything that indicates that she can perform magic while she is dead.