first of all he's not on mirage island, it's birth island. and yes u do. also if u didnt already know this, u can only get to birth island by cheating with a cheatcode device.
When you get to mirage island you will find ONLY wynauts.
you can find a skill tag on mirage island.
There is no "easiest way" just do the normal way. Normal way: Talk to the old man in Pacifidlog Town. Once he see's it go to route 130. Look for mirage island. Note: Mirage island is not that important it only has the rare Leichi Berries and in the grass all you will find is wild Wynat.
you can find deoxys in the map 10 form a letter l move and bingo there it is deoxys
In 9th island in tall grass you can find deoxys
No.
You can only find Wynaut on Mirage Island in Pokémon Emerald.
go to a house in pacidolog where two men talk about mirage island the second guy randomly tells you if the mirage island appeared or not (if he says theres no mirage island you should wait until the next day) if he says that theres mirage island surf east (dont go to the fast currents take the other side) then in mirage island you well find wynauts n the grass and a really rare berry which you can receive only in mirage island
you find mirage island outside pacifidlog town when you have a mirage sensitive pokemon. you have to talk to an old man in the southeast corner of town without a hat. if he says "i can see mirage island today!" then then you have a mirage sensitive pokemon and can visit mirage island on route 130 or 129
When you trade Deoxys it becomes its normal form
Mirage Island appears when you have got a Wanuat. Sometimes Mirage Island appears and sometimes it doesn't - the old man will normally say " I can't see Mirage Island today" but rarely he will say he can - and that's when it appears. DO NOT let your Wanuat evolve otherwise you will not find Mirage Island.
Sadly, the only thing you can catch at Mirage Island is Wynuat, but you can find lots of rare Berries and Items there.