Yes. The new Ruby and Sapphire are upgrades of older versions. You may do all of the things in which you can do in regular Ruby and Sapphire, but they've added things (I don't know what those things are yet).
Yes, you may battle Stephen Stone, but I don't know if he will have EXACTLY THE SAME party or not. He specializes in steel type, so its possible that the company might have added someone like Ferrothorn.
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.
You get the Eon Flute from Steven after battling Kyogre (After getting the 7th gym badge.) Using the Eon Flute allows you to soar.
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.