I have to correct you on that last part, having recently attuned an alt all the world bosses that have an achievement attached to them (which means 90% of them bar Zoetic and a couple more) do count for attunement retroactively.
You can even do them pre-50 and they will still count.
When you get to that step you see the quest progress instantly to 33% or 66% or however many (out of 12) you had got.