Council of Elrond

Rings played with these cards are not considered in play (btw: so I cannot discard them to fulfill the requirements of Rolled down to the Sea, right?) unless for the purpose of uniqueness, is that true?

It just came to my mind that if I'm the lucky starting player I can immediately lay e.g. Dwarven Ring of Durin's Tribe on it and prevent my opponent from playing it. Has he any chance to get his own copy out into play?

Dark Minions, Placement of Cards “off to the side” (Clarification)

(...)
Cards placed off to the side are in play for the purposes of uniqueness. Unless stated otherwise, when a host permanent-event is removed from the playing surface, any cards placed off to the side under it are discarded.
(...)

Has he any chance to get his own copy out into play?

As long as your unique copy is placed with Rumours/Whispers no?

P.S.
Play Rumours of Rings with Crown of Flowers and give him chance of playing Twilight. Host cards of cards placed off to the side can be targeted normally.