Hello there,
Long time Atingo farmer here and pet seller
You haven't got the necklace because you were in party. Necklace doesn't drop if person that landed the last hit to the pet was in party, so yeah it is intended for solo players.
Atingo can be dragged anywhere as it doesn't have reset range, this is intended for a safe kill and not be jumped by half the server.
When Atingo despawns it means it will appear in another location, happened many times...sometimes luck is a factor.
The chance to spawn a pet is 1/10, meaning 90% of the time you'll see only sin eaters.
Hopefully this information helped.
Cheers,