You can only activate one code per account, and once a code is used, it will no longer be accessible. All of the codes are on a first-come-first-serve basis, and once they are gone, they are gone ...

You'll then get redirected to a page where you can click a link that takes you to your Steam code. From there you need to open Steam, click Games, go down to Activate a Product on Steam, and ...

3) Almost every time I run the game, Rockstar Social Club asks me to recover the activation--I'll get a screen that prompts me with something called a "request code" and I just hit the back button ...

Select Library, and a page will appear with the words Activate a product on Steam. Select Add game, and a text box will appear, allowing you to redeem your code.