Sanctaphrax's definition still works when taking that link into account. It's still a fact external to the character, that the character is the only one to catch on to. It's all just different readings of the same material and it works both ways. I'd be inclined to run it like Sanctaphrax, because I like to encourage my players to take control of the story.
Character aspects should never be declarations, they are either forced on the character by a maneuver or they are discovered with an assessment/guessing action. You could for example declare, that the old man in front of you hasn't used his shotgun in a while, giving it the aspect "rusted", but you would not be able to declare "untrained" on the old man. Of course, if the man has an aspect like "gun nut", the declaration of the rusted shotgun could simply fail by veto of the GM.
Actually you can make declarations that are character specific. One of the examples that they use in the book is that of a thug having a bad knee. Of course these kind of aspects are a little more risky as the GM is a little more likely to have an image in mind of what they are and is thus more likely to tell you no. A PC is the only time I would agree with you. I have always felt that a player's control over their character is sacrosanct, so declarations by others about their character are strictly controlled by the player.
A goblin gets obliterated by the Wizard. The Wizards ally turns to the other goblin and says, "I make a declaration that the goblin is shaken up by the death of his companion." (he wants to tag it for a +2 or for effect to have the goblin flee)
Is that a fair declaration, or would that be more like an Intimidation Maneuver?
Maybe it's an Empathy declaration to read the Goblins reaction to the death of his buddy?
I'd say it could be either, and it's up to you (the GM) to make that call. Do you think the goblin would be shaken? If not then make the declaration difficulty high, or say no and tell them it's an intimidation maneuver.
Can you tag declarations for effect? "with my free tag, I think the goblin will trip over that root." (this would be a compel on the goblin)...
A declaration is an aspect. It can be used in any way an aspect can be used. There's some debate as to whether a tag can be used for a compel (even within evil hat, as I have heard different things from different developers), but the short answer is yes...kinda.
I've been kind of judging that if it makes sense that someone should be able to resist, then it's a maneuver.
There are maneuvers that are unresisted though. Scene maneuvers (like flipping a light switch) are usually against a difficulty set by the GM. The rule of thumb I usually use is "Is a character acting to create an advantage (Flipping a light switch, scaring a goblin), or are they taking advantage of an existing situation (the scene is dark, the goblin is scared)?" The maneuvers are a little more reliable because the character is acting to make it so, but declarations take less effort to use. And of course this is all up to the GM to decide whether the declaration is appropriate (IS the scene dark?) or whether the character needs to act to make it so.