After you complete the Delta episode after beating the Elite Four, you will receive a ticket from Norman to go to the Battle Resort. Take this ticket to either Slateport City or Lilycove City, where they will examine your ticket and you are able to ride in the boat to the Battle Resort.
You battle Kyogre for your chance to capture it in Pokemon Alpha Sapphire after you capture the groudon.
You can buy a Sachet at the Battle Resort for 32 BP, which is the only source to get a Sachet.
You can buy a Whipped Dream in the Battle Resort for 32BP. This is the only place where you can get a Whipped Dream.
The Reaper Cloth can be bought in the Battle Resort for 32BP. This is the only place you can obtain a Reaper Cloth.
There is no roller skate item in Alpha Sapphire.
Metapod can only learn Harden after evolving or capture, but it can learn Bug Bite, Electroweb, and Iron Defense by taking it to the Move Tutor at the Battle Resort in Pokemon Omega Ruby and Alpha Sapphire.
You can catch Skitty in Pokemon Omega Ruby and Alpha Sapphire in the barks of trees.
As of Pokemon Omega Ruby and Pokemon Alpha Sapphire, you can take Staraptor to the Move Tutor in Battle Resort and teach it Sky Attack. For Generation IV, you can teach Staraptor Sky Attack from the Move Tutor in Pokemon HeartGold and SoulSilver.
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.