The main point of contention may well be that I feel it is possible to roll such a successful declaration/skill check, while not beating the veil itself and you do not agree.
Pretty much.
The reason I think this is that the veil is only making the caster, not all of his external actions, undetectable.
The veil strength is a block against the target being detected. It is not a block simply against being seen and positively identified. I feel external actions are certainly detectable, and grounds for a declaration to help the alertness roll to beat the veil, or a compel to make the veil fail, but without beating the veil's strength with an alertness roll, the guard in question simply does not know that there is someone there, and where that someone is.
Someone taking obvious actions should certainly strain the veil's ability to hide them, or be grounds for a compel along the lines of "There's only so many places a person can be standing while they're doing that, he guessed right." But unless someone's Alertness roll succeeds or there's a fate point being handed over, an intact veil means the people looking don't know where you are.
Add to this the fact that directly perceiving someone is not the only way to know where they are and it should be possible to make a successful declaration of "I know where he is" without taking an action, and with a lower difficulty than beating the veil.
A successful Alertness roll would not be its own action, but a prerequisite for direct action. You're right that directly perceiving someone isn't the only way, but that's not the only way to narrate a successful Alertness roll either. The skill check to make the statement, "I know where you are"
is the veil's strength.
Back to the example of Molly's first veil vs. Murphy, Murphy didn't see Molly, and Molly was, in fact, invisible even after she'd been detected. Murphy made the Alertness roll not to directly see Molly, but to discern where Molly was standing.
You seem to be hung up on the idea that an Alertness roll means you literally see through the veil and directly perceive the person, which isn't necessarily the case. When someone makes the Alertness roll to beat Molly's veil, it doesn't mean Molly suddenly pops back into sight, it just means the veil wasn't good enough to keep her from being found.
A successful Alertness roll would mean, "I know where you are." Anything less, and the veil is doing its job at keeping the person hidden. They might be suspicious as hell about the self-opening doors and floating candles (and this suspicion should be tagged and compelled to make the veiler's life more interesting), but they can't pinpoint your position without making the Alertness roll.
It's a simple dichotomy--Alertness success: I know where you are; Alertness fail: I don't know where you are.
This is just a long way around to exactly what I had before, which was in effect allowing people who could justify making a good guess as to your location to attack you. Since the declarations like these are trivial common sense things (in the example I was using), and they do not take an action, there is no need to jam the mechanics in (and I never though of describing it this way).
Except that discounts the veil's strength. In this example, someone could have a 10-shift veil, against someone with an Alertness of 2, and then still be found and attacked without a compel.
But if you really want a mechanical justification, just say that when you opened that door, the guard makes a declaration, with alertness, of "Gotcha" against some difficulty (if you want, it could be against a stealth roll, to open the door sneakily, if that makes sense in this situation, or it could just be a 2 or a 3), and then tags it for effect to allow him to attack. It is just a round about way to the same thing.
I could see having the caster make a Stealth roll in defense against the declaration, yes. (I see an exchange like "The guard sees you do that" "What if I wait and do it quietly while he's not looking?" "Roll Stealth and see if you can manage it, then.") But if that is then going to be tagged to allow the attack--despite the guard not making the Alertness roll--the caster really deserves a fate point because that's a compel.
You could argue that this declaration would need to beat the veil strength, but I argue, that when a veiled people takes an action that can be perceived by those outside, they should not have to beat the full veil strength to declare that they notice.
To notice the action, or result of the action is one thing. To immediately conclude, "Right there is someone there I need to shoot," and then successfully cause stress is another.