The fourth Gym is in Lavaridge City on the most western side. The Gym Leader inside is named Flannery and she specializes in Fire-type Pokemon. Her Pokemon are a level 26 Slugma, level 26 Numel, and a level 28 Torkoal. After defeating her, she'll give the TM Overheat with an addition of the Heat Badge for your Badge collection in Hoenn.
The fourth Gym is in Lavaridge City on the west side of the city. The Gym Leader is named Flannery who specializes in Fire-type Pokemon.
No, you will need to trade for them or transfer them via Pokemon Bank to get them in Pokemon Omega Ruby and Alpha Sapphire.
You can buy a Sachet at the Battle Resort for 32 BP, which is the only source to get a Sachet.
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.
You can buy a Whipped Dream in the Battle Resort for 32BP. This is the only place where you can get a Whipped Dream.
You battle Kyogre for your chance to capture it in Pokemon Alpha Sapphire after you capture the groudon.
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.
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.
On field, it'll attract Pokemon to you. In Pokemon X/Y/Omega Ruby/Alpha Sapphire, you can now attract hordes of Pokemon. In battle, it decreases your opponent's evasiveness.
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
It is not yet known if there is a building location for the GTS in Pokemon Omega Ruby and Alpha Sapphire.
Yes, all the TMs that were in Pokemon X and Y will be available as TMs in Pokemon Omega Ruby and Alpha Sapphire.