Cheating device.
The Swarm Pokemon are post-Elite Four Pokemon which appear in random Routes and Caves. These Pokemon are rare and difficult to obtain otherwise. Once the day is over, another Swarm Pokemon will appear somewhere else.
Once you have defeated all the gym leaders and elite four they will randomly appear
It could only appear once since its so rare but it could appear more than once
Yes, you can duplicate Pokemon on Emerald more than once.
Uxie is indeed a rare Pokemon appearing only once in the games it can be caught in. Uxie is part of a classification of Pokemon called Legendary Pokemon. Legendary Pokemon are the rarest Pokemon and can only be caught once per game they appear in.
Not In your party at once
Electirizer is sometimes held by wild Elekid which only appear in Pokemon Pearl if you have a copy of Pokemon FireRed. To make the wild Elekid appear you need Pokemon FireRed in the GBA slot of the DS. Once this is done wild Elekid should start to appear on Route 205 and the Valley Windworks.
Once a legendary Pokemon has fainted it won't come back until you restart your game. This is assuming that you are talking about wild Pokemon and the ones that only appear once in the entire game.
Yes, you can. In fact, that's where I train my Pokemon when I don't train at Mt. Battle. And any shadow Pokemon you might have missed from him will still appear when you battle him again.
There is no sleeping Pokemon in the Tanoby Chambers on Pokemon FireRed. However Unown do appear in these Ruins once you've solved the Tanoby Key puzzle.
No, even with the event.
Once you kill Tornadus, you have two chances. Since it is a legendary Pokemon, there is only one of its kind, and will appear after the first one.