You cannot stab him in his long sleep...

[Konrad Klar]

Any card placed off to the side absolutely cannot be targeted or otherwise affected by the game except by cards that specifically affect cards placed “off to the side.”

This gives some chance of survive for unique dragons placed with Bring Our Curses Home, Foes Shall Fall etc. if their other manifestation has been defeated.

[Bandobras Took]

There's no particular problem with that, since the dragon in question could never attack (it would never be playable if a manifestation of that dragon has already been defeated).

That is an interesting situation, but I can't think of any exploits.

[Konrad Klar]

[...]If at any time an
attack from a manifestation of a unique Dragon is defeated or if the manifestation is
otherwise removed from the game:
• All existing manifestations in play of the same Dragon are removed from the
game.
• No further manifestations of the same Dragon may be played.

Underline mine.

At least in case of Bring Our Curses Home, Foes Shall Fall, the dragon was already played earlier and is not played again when it attacks.

And even if its different manifestation would not be defeated, the dragon could not be played again from obvious and simple reason - it is already in play. So defeating its manifestation does not make the question of its playability worse (it is still: whether it could be played there if at all).

[Bandobras Took]

The creature on such cards can only attack if it is playable. Playability is not the same as keyability, even though keyability is included in playability. For example, if I have Gandalf in play, then Gandalf the White Rider (hazard) is not playable. Similarly, if Gandalf has been eliminated, Gandalf the White Rider is not playable.

BOCH and FSF do not make you play the creature, true, but they do check if the creature is playable. Once a manifestation of an entity has been defeated, no other manifestations of that entity are playable.

[Konrad Klar]

The creature on such cards can only attack if it is playable. Playability is not the same as keyability, even though keyability is included in playability. For example, if I have Gandalf in play, then Gandalf the White Rider (hazard) is not playable. Similarly, if Gandalf has been eliminated, Gandalf the White Rider is not playable.

BOCH and FSF do not make you play the creature, true, but they do check if the creature is playable. Once a manifestation of an entity has been defeated, no other manifestations of that entity are playable.

If Gandalf the White Rider is in play as permanent-event it is not playable as creature even if it could be keyed to site or region.
Right?

If so, question is whether Gandalf the White Rider may be considered playable if its copy is placed off to the side with Bring Our Curses Home?
No one can play Gandalf the White Rider as long it (or its manifestation) is in play. Just its presence in play prevents from playing it.

So in such situation, can unique creatures placed with BOCH and FSF attack at all? After all they may be merely keyable to site or region, but cannot be played because they are already in play.

[Bandobras Took]

Well, you got me. I've always interpreted the cards as "playable as if the creature were in your hand," but that isn't actually what the card says. Drat.

[Konrad Klar]

[...] I've always interpreted the cards as "playable as if the creature were in your hand," but that isn't actually what the card says. [...].

...and possibly it should be clarified in such way (something for NetRep Team).

There are other factors that could prevent the creature from being played even if it is keyable and its manifestation has not been defeated.
For example not revealing the creature in question in result of Spying Out The Land.

EDIT:
Spying Out of The Land -> Spying Out of The Land