You need the 8th gym badge. You obtain the HM itself in the Cave of Origin.
there is a key in the solar cave then use it on a big wall with a key hole in it there is a key in the solar cave then use it on a big wall with a key hole in it
You need to get the 8th badge and the HM for Waterfall
In Ice Cave.
In ice path, next to Blackthorn City.
There is no roller skate item in Alpha Sapphire.
You get it from Juan in the Sootopolis Gym after beating him. Then, you get Waterfall.
You get the HM waterfall in the same cave you get Kyogre
there is none
HM 9 is Waterfall.
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.
You have to dive into the sea and go into a tunnel underground and you will find the waterfall there.
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.