"If you are receiving the SoR, is the lvl 80 boost only available for your current characters?"

Correct. The character has to exist upon accepting the SoR.

Incorrect. I recently scrolled one of my own defunct accounts. Accepted the scroll, logged in and mucked around, then went to battlenet to claim the 80. It explicitly told me I had the option of making a new toon before I chose to super-ding. I did so, and now have an 80 of a class I've never rolled before.

Sidhebane is correct here.

I sent a SoR to an old account I had used for RAF several months ago. This was before RAF let you go to 80, so the only characters on that account were around 60, and BC wasn't even added on.

I accepted the SoR on that account, let it on the page where it allows you to activate the bonuses, logged in and made a new level 1 Shaman. After logging out, I went to apply the benefits, and my new level 1 Shaman was on the list, and allowed me to boost him to 80 and get all the tasty stuff that goes with it.

On a side note - and rather sad - this new level 80 has better bags and flying speed and half of my 85s.