From what I am aware of, previous content normally becomes free after a certain period of time. For the sake of buying something you may already have, I suggest checking the Bungie store via Bungie.net just in case! Hope this helps! 💜
Yes pretty sure you can.
Yes and warmind and Osiris are free to play with new light
I think so... I know with Destiny's "New Light", the "Curse of Osiris" and "Warmind dlcs are free to play now...