As someone else mentioned, even the RCC doesn't require anyone to believe in apparitions or their messages. You can be a faithful Catholic and believe it's all bogus. Even from their perspective, then, basing the truthfulness of a doctrine on a purported vision is wrong. The IC was declared in 1854, while the Lourdes visions occurred in 1858: one might argue that the vision supports the doctrine, but the reason one believes it in RCism is because it was infallibly defined by Pius IX, not because Our Lady said so to an uneducated peasant girl.
Catholics, Orthodox, Protestants, Hindus, etc. all have "miracles". Do they come from God? I don't know. From the devil? I don't know. In the end, I think there is wisdom in simply ignoring them, at least when it comes to matters of faith. The faith is evaluated on its own terms, not because some girl seeing visions, or some monk with the stigmata, or an icon weeping myrrh indicates to an individual that it might be right.
I didn't know the "IC declaration" was made at Lourdes on 25 March. In light of this, your hypothesis about the true meaning of the Virgin's words is interesting.