isn't it stated only Dragon born can use Dragon shouts? all nords are Dragon born?
to use Dragon shouts or to get the ability to learn them you need to defeat a Dragon right?
Dragons are just now returning and Thu'um has been around since the nords came to Tamriel
How can Thu'um be Draconic when the Language of Dragons has been unknown? as well as the Origin of Thu'um
1) No it isn't. Dragonborn are just much, much more powerful in it. It was stated that Draugr and the Grey Beards can use the Dragon Shouts, and since you are the last Dragonborn, they aren't. I'm making a note here: Dragon Shouts, not Thu'um.
2)No you don't. At least it was never confirmed. It was stated that you defeat the dragon, then you get the ability. But it's not like you couldn't get it otherwise.
3)&4) As you say,
Dragons are just now returning
They were around thousands of years ago (supposedly in Atmora, since there's no single sign of so genocidally hostile dragons in lore), that was when the Nords have learnt the Dragon Language and created the art known as Thu'um. Then they were driven away, and now they are returning. And stuff like that the powerful Tongues couldn't speak without causing destruction could easily be just legend, or be made just a legend to put Dragon Shouts into the lore
And again, I remember quite well from one of Todd's interviews that he has called the Dragon Shouts "the Thu'um". There were very many of them, so you will have to watch all of them again yourself, or just trust me. :foodndrink: