There's a significant difference between being touched by Darkin Magic and actually being a Darkin. I'd say that Tryndamere has been touched by Aatrox's magic and that's how he can... well... ignore the status effect that is DEATH. Likewise, I wouldn't be surprised if Varus's bow is a Darkin bow, but that doesn't actually make him a Darkin. It may well be that the Pit of Pallus is a prison for a Darkin, or perhaps it is just a storage facility for a Darkin named Pallus's bow? But again, that doesn't make Varus himself one.
Jax and Eve could be Darkins, but they don't have the right anatomy for it, though Jax is altogether too good at fighting. I suppose though that were he a Darkin, there'd probably be some difficulty plying him from his weapon, so I doubt a Darkin would settle for a lamp post.