1) Monarchy and Gunpowder can obsolete an ancient and an ancient\medieval wonder respectively. That means that I can use it towards another player's wonder?
2) Mass Media state that my culture event cannot be cancelled, regardless of other game effects. As I understand it and after reading some other posts about mass media and UN, if you have mass media you can throug ha culture event to a palyer with the UN wonder. Is that valid?
1) Yes. No need to obsolete your own wonders you can just overbuild them. They are definitely for using against other players and their wonders.
2) UN makes the player unable to be targeted by events (without permission) so you can't target him with an event. If you successfully target someone then they cannot block (cancel) the effect from taking place.