I asked this question on Blizzard forums a few weeks ago with no response, but since the patch is now live, I have my answer.
For those wondering, you need to log each character on for their achievements to copy to the account level... thus if you have something precious on an inactive account... time to spend $15.
Still have no idea what happens if you delete a character or inactivate the account again, but would assume they stay copied.
Everything that becomes account wide will remain on your account regardless if you delete the character that originally earned it, I'd assume the same would hold true even if you deactivate your account again.
it sucks i can't wear my "of the alliance" title on my twink when he earned the most hks