Can this be stolen?

yes, because, like Pyat Pree, it is a claim replacement, and, therefore, affecting the claim mechanic, which is, in turn, allowing you to take control of an opponent's location instead of normal claim (that's the important wording of the card).

Also, the kicker... as I understand it, it is still protected from (now your opponent's) non-plot card effects, since it never left play. I may have that wrong... just depends on whether the control change affects the protection like leaving play would.

Yes. And it maintains its immunities.

The reason for this is that while it's immune to opponent's non-plot effects, it's *not* immune to claim (so claim replacements can affect it, just like Pyatt can Kill TRV). Other similar effects include Victarion's Reavers.

The look on my opponent's face when I stole his Longship Grief in a Mirror Match...now I'm running his agenda and mine too...

I felt bad...

The look on my opponent's face when I stole his Longship Grief in a Mirror Match...now I'm running his agenda and mine too...

I felt bad...

For breaking the rules ? 3.29: You may not play, put into play, or take control of a unique card already in play that you own or control

(Except for dupe'ing). If you had Grief in play, you couldn't steal his Grief, unfortunately.

The look on my opponent's face when I stole his Longship Grief in a Mirror Match...now I'm running his agenda and mine too...

I felt bad...

You should feel bad!!

It was a good play, though...grumble grumble

The look on my opponent's face when I stole his Longship Grief in a Mirror Match...now I'm running his agenda and mine too...

I felt bad...

For breaking the rules ? 3.29: You may not play, put into play, or take control of a unique card already in play that you own or control

(Except for dupe'ing). If you had Grief in play, you couldn't steal his Grief, unfortunately.

Because it was a mirror match, I had no few Captains and Raiders of my own, so his HOD-Protected Grief plus my own Black Sails meant that the game was more than a little unfair at that point, as Clique84 is right to point out.

I always feel bad when my Greyjoy decks work well...

For breaking the rules ? 3.29: You may not play, put into play, or take control of a unique card already in play that you own or control

(Except for dupe'ing). If you had Grief in play, you couldn't steal his Grief, unfortunately.

Then what happens if have Breaking and Entering flipped with The Iron Throne on table, and i discard my opponents Iron Throne via claim? Do i dupe my own throne or discard it?

I hadn't really thought of using BftSI that way, but it's a nice bonus against HoD for sure.

You must own and control both the card you are duping and the card you are duping it with. So if you pull an opponent's copy of a unique card you already have in play with Breaking and Entering, the card you pull is discarded (because putting it into play would break the rules).