If I remember correctly, it actually took 18 attempts to resurrect her, not 5. Her phasing abilities are the reason why the previous 17 attempts didn't work.
From what I have seen online, people have taken this to mean that her phasing abilities are also the reason why she can't pass through a gate. So far no answer has been given in the comics and she has yet to try to use a gate post resurrection.
Yes. 18. 18 is a very significant number in Judaism. The number signifies life. "Chai," the Hebrew word meaning "living," is actually a combination of the 10th and 8th letters of the alphabet.