In Pokemon ORAS, I want to SR for shiny legionaries, but I already caught almost all of them, and I wanna be able to SR for them, so my question is, if I release all the legendary Pokemon I have, and then beat the elite four again, do they re-appear?
No. Once you catch a legendary Pokemon it will not re-appear, even if you release it. They only re-appear after defeating the Elite Four if you caused them to faint rather than catch.
While not an ideal solution, you could deposit all your Pokemon into the Pokebank app (if you've already used your free trial period, you will need to pay for access which is ~$5 for a year), and then restart the game. As long as you are on the same system/Nintendo Network ID you should be able to withdraw all your old Pokemon from the bank onto your new save. This does mean you would need to re-complete the entire story again - but you have another shot at catching all the legendaries and soft resetting for shinies.
No, you can't. They re-appear only of you defeat them.