Yes, the grieving process isn't predictable. No, Ianto isn't following Kubler-Ross -- notice he hit "denial" in 04 before "shock" in 05. I doubt the writers will prolong his stages though. He still has "negotiation" and "acceptance" left to go.

In my long fic "Training", I'm basing Ianto's reactions on my personal experience as the primary caregiver of a severely disabled loved one who died, not on Kubler-Ross.