Whoa, it sure looks like summer in here.
1. Shiki only cut Arcueid, he never knifed her on her point of death, that's why she was able to regenerate.
2. Shiki doesn't need to grasp "immortality" to kill someone/something. He simply needs to grasp the concept of "death" for that certain someone/something. In that way, C.C./V.V.'s immortality is no different from a Dead Apostle. He would be able to kill them with ease.