After you complete the Primal Kyogre or Primal Groudon narrative, you can go to the left of the Pokemon League entrance to obtain the Mewtwonite Y.
Mewtwo cannot evolve, but it can mega evolve temporarily in Pokemon X, Y, Alpha Sapphire, and Omega Ruby by letting it hold a Mewtwonite X or Mewtwonite Y.
You can catch Skitty in Pokemon Omega Ruby and Alpha Sapphire in the barks of trees.
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.
These Pokemon can be found before the National Dex: Latias (Alpha Sapphire), Latios (Omega Ruby), Kyogre (Alpha Sapphire), Groudon (Omega Ruby), Regirock, Regice, and Registeel. These Pokemon can only be obtained in the Delta Episode: Rayquaza and Deoxys. Post-National Dex Legendary Pokemon are: Regigigas, Ho-Oh (Omega Ruby), Lugia (Alpha Sapphire), Heatran, Mesprit, Uxie, Axelf, Palkia (Omega Ruby), Dialga (Alpha Sapphire), Giratina, Cobalion, Terrakion, Virizion, Reshiram (Omega Ruby), Zekrom (Alpha Sapphire), Kyurem, Raikou, Entei, Suicune, Tornadus (Omega Ruby), Thundurus (Alpha Sapphire), Landorus, and Cresselia.
No
It is not yet known if there is a building location for the GTS in Pokemon Omega Ruby and Alpha Sapphire.
The ability of Mega Diancie in Pokémon Omega Ruby and Alpha Sapphire is Magic Bounce.
Yes, all the TMs that were in Pokemon X and Y will be available as TMs 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.)
all of them
There is none