There's no limit on how long you have to wait before you can enter another Mystery Gift code. If you got, for example, a Mew and a Darkrai code card, you can enter them both right after one of them.
No, you will need to trade for them or transfer them via Pokemon Bank to get them in Pokemon Omega Ruby and Alpha Sapphire.
No, there are no roms for Pokemon Omega Ruby and Alpha Sapphire currently nor a working 3DS emulator. (They are also incompatible with the Desmume emulator.)
You have to transfer it from your Pokewalker. You get it on Icy Mtn. Road.
It evolves by happiness
You can get two ferry tickets:The ticket for the SS Tidal is obtained by completing the Delta Episode.You can get the Eon ticket by Streetpassing someone with Pokemon Alpha Sapphire or Omega Ruby.
There is no roller skate item in Alpha Sapphire.
You can catch Skitty in Pokemon Omega Ruby and Alpha Sapphire in the barks of trees.
You battle Kyogre for your chance to capture it in Pokemon Alpha Sapphire after you capture the groudon.
Magikarp
Yes, Tm's in Pokemon Omega Ruby and Alpha Sapphire are re-useable
No, you will need to trade for them or transfer them via Pokemon Bank to get them in Pokemon Omega Ruby and Alpha Sapphire.
Houndour can be obtained by trading from Pokemon X or Y. It cannot be caught in the wild in Pokemon Omega Ruby and Alpha Sapphire.
Beat the elite four again(sucks right?).
You will need to trade for Weedle or breed Kakuna. Kakuna can be found only in the Safari Zone in Pokemon Alpha Sapphire and Omega Ruby.
No
After receiving a Pokemon from Mystery Gift players need to go inside any Pokemon Center and should find a lady dressed in blue clothing waiting for them. Speak to her and she will reward the player with the Mystery Gift Pokemon. Players will need at least one empty slot in the Party in order to receive it.
It is not yet known if there is a building location for the GTS in Pokemon Omega Ruby and Alpha Sapphire.