As a Forced Reaction at the beginning of a phase, Weapons at the Door must happen before any non-forced reaction to the phase beginning - for instance, if a Milk of the Poppy is attached to your Brothel Madame, the Milk would be back in hand before the Madame's reaction window, meaning you could trigger the Madame. As for other forced reactions? If they conflict, it falls into "first player decides" territory.
Whereas Risen from the Sea gains the attachment card-type but does not have it printed, a duplicate of a unique attachment loses the attachment card-type but does have it printed. What this means is that, when the forced reaction triggers, if you control a duplicated unique attachment, you have an interesting situation. In essence, both the card and the duplicate are printed attachments. However, as the Interrupt happens before the cards bounce, you can still choose to use the duplicate to save the original (as with First Snow)… or, you may return not only the unique attachment to your hand but the duplicate as well (unlike First Snow).