The speculation making rounds is that Keening has lost a fair some of its power after Lorkhans heart was unbound, and the Dovahkiin is capable of wielding it without any ill effects. It was supposed to be wrapped up properly upon transport, indicating it is still dangerous to handle. How Arniel could wield it is a different matter though. The reason you can summon him is thought to be the cause of him replicating the disappearance of the Dwemer. Like them, he (Unwittingly) bounded his spirit to the closest divine being, in this case, a Dragonborn.
EDIT: Keening being in Skyrim should be self evident. How it left the hands of the Nerevarine is a different matter, but lets face it, artifacts of immeasurable power don't stay in anyones hands for too long.
1. Keening's in Skyrim because Arniel made a significant effort to have it shipped to him from morrowind. The real question is why did the Nerevarine lose it.
2. This is disputed, last I checked. Arniel makes reference in his dialogue to the fact that the Dragonborn wasn't struck dead by holding it barehanded, so we can safely conclude that the mortal wound effect not killing the player is not a bug. This leaves us with a few possible reasons for Keening being able to be safely handled. One possibility would be that it's enchantments have weakened in the 200+ years since Morrowind. Another possibility is that Keening's lethality was a side effect of it being used on the Heart of Lorkhan, rather than a direct consequence of the enchantments, and that lack of exposure has caused this side effect to wane.
3. He bound himself to you like the Dwemer bound themselves like Numidium. A successful experiment on his part.
With Skyrim I ment the game.
Hm, after thinking about it might have lost most of its power due to the fact that the heart was destroyed.
In my eyes this was just another way of Bethesda to bring some old things back (Jiub
).
One thing that always bothers me is that the Nerevarine is hardly ever mentioned after Morrowind.
Maybe I just overread some books but I think it's true that nobody knows what happened to him/her