Also, being a Then-effect, the power grab part only happens if the save part was successful.
But the "attach as +1" part of Risen from the Sea is a "then" effect, too, and we know that works to save characters from terminal effects. So I'm not sure how the power grab being a "then" effect is relevant (which, apparently, you don't believe either since your "Devious Intentions" example depends on the "then claim power" happening).
Personally, I think you'll need an official ruling on this because the "remove from the terminal state" rule is not interpreted solely from the point of view of the effect as it resolves. It also includes the game state once it does resolve. For example, if I put Flame-Kissed on your Maester Wendamyr, you can't kneel him and use his ability to save himself because it doesn't remove him from the terminal state. However, if Stoic Resolve is one of the active plots, you can kneel Wendamry to save himself because, once knelt, the continuous effect making knelt characters "cannot be killed" removes him from the terminal state. The save does not directly remove the character from the terminal state, but it indirectly places the character into a game-state situation where the terminal effect no longer applies.
So we know that the rule about "the save must also remove the character from the terminal state before it can be triggered" is satisfied when the save only removes the character from the terminal state if it is combined with active continuous effects. Well, if you think of "the game is over when a player reaches 15 power" as a continuous effect (which isn't an unreasonable thing), there is a really strong argument that this save, combined with a continuous effect, puts the character into a game state where the terminal state no longer applies - because the game is over.
While I tend to agree that when checking the play restriction on the save, you must assume the game continues (and therefore, "game over" is not an acceptable game state for removing a card from a terminal state), it's not unreasonable or illogical to go the other way.
To my mind, both positions are equally valid, so an official FFG ruling is probably necessary.