2. I would say it can only be used to advance the nation not activate it. Wormtongue quite clearly states that only the Fellowship and Companions can activate Rohan.
3. If it's actual combat that's initiated, he can of course retaliate - a battle starts after all. But special attacks like Ents, Dreadful Spells, Dead Men, Faramir' Rangers, etc. don't allow to fight back.
1. Can the same elven ring be played/used by the SP immediatly after he had received it from the FP in the same turn?
Just to clarify, the Shadow player must wait until his/her turn to use the Elven Ring. It may not be used to change a Shadow Action die to an Eye during the Free Peoples player's turn before the Free Peoples player can move the Fellowship through the use of the Elven Ring.
"War is much more fun when you're winning!" - General Martok