In Trumps of Doom why didn't Fiona just tell everyone who Luke really was? Even if you argue that she wasn't sure about Merlin there was no reason to keep the others in the dark.
Immediately after seeing a picture of Luke, she disappears for one and a half books. By the time she reappears, it's public knowledge. Her disappearing is probably a gambit, rather than simple self-preservation. Rinaldo went after his father's killer first, as well as one of his father's betrayers. By leaving Amber with Bleys, she can try to draw him into a trap. Telling the others means telling Random, who might order her otherwise.
And she couldn't be bothered to leave a simple note? Because of her Merlin was trapped and the Ghostwheel was nearly taken over.
In the first series, Corwin reveals a substitute for gunpowder. We later learn that a number of these weapons and their ammo are left in Amber's armories to provide for its future security. Yet in the second series we learn that an enemy of Amber apparently discovered the origin of this gunpowder-substitute and has been mass-producing it. It doesn't seem likely that this person just started bringing samples of every pink powder in every universe to Amber and testing them for explosiveness... is it really plausible that Benedict, who had enough data to infer the nature of the powder, wouldn't have taken precautions - fairly extreme ones - against any of the stuff being taken from Avalon by anybody outside the family?
Everyone in the family, including Brand, knew where Benedict was staying and whose residency it was before that. Everyone in the family also knew that Corwin and Benedict had some sort of severe falling out immediately prior to Corwin arriving in Amber with his own rifle battalion. As far as precautions, Benedict's attention is elsewhere during the war, and afterwards he's in Amber. No doubt he has spies in Avalon, but they're no more omnipotent than he is, and security may have relaxed without the constant threat of the hellmaids. Also, considering that the rouge originally came from Corwin's Avalon, and that Benedict's Avalon is a shadow of that place, who's to say that jeweler's rouge from a shadow of that shadow wouldn't have the same effect?