Kai said hes made the spell so that any attempt at loophole will just kill Elena. Maybe Alarics daugthers could absorb the spell but Bonnie stopping her heart for a minute or becoming a vampire would be something Kai considered. I just realized even if Bonnie really wanted to become a vamp to stay with Enzo she might have to risk killing Elena LOL

Siphoners are not witches, so being able to steal power to temporarily become something you weren't supposed to be is a loophole. They in fact become witches as long as they can siphon magic which is why heretics remained witches after turning as they are now siphoners with a constant source of magic. It's no different than Kai becoming a witch after taking his siblings magic. He became a full witch, loophole.
Nemo Unus Animabus Carnem!

no heretics are the loop hole. siphoners are just witches without active powers unless they siphon magic. they were born that way.

Siphoning is a loophole. Siphoners are not witches, they are siphoners and as siphoners their is a loophole that makes them witches after they have siphoned and also allows them to become full witches after being turned, those are loopholes. Kai knew a group of siphoners were coming to town I would imagine he planned ahead. Elena life is directly tied to the magic keeping her asleep, so wouldn't siphoning off the magic also siphon off her life force killing her? I mean that's how a nutcase like Kai should think when casting such a curse.
Nemo Unus Animabus Carnem!

Valerie or Alaric's kids can easily siphon away that sleep spell. siphoning isn't a loop hole, its absorbing magic/spell and redirecting it somewhere else. just like how Kai absorbed the magic barrier and werewolf venom. the writers wants us to forget about that.

I can see Bonnie deciding to become a vampire in order to spend the rest of her life with Enzo, or Enzo turning Bonnie to save her from dying. However I do not know if that would work since Kai said no loop holes. But Kai might have just said that so they can avoid trying.
Lilly is the one who suggested that spell to Kai, so wouldn't Lilly (or the heretic) have known how to reverse it?
[love]
Kades! [/love]