I recently got to level 38 with my character, but somehow in a chest in Honeyside I have a Daedric Greatsword. I don't even remember how I obtained it..
well you don't need to do the greybeards mission for you to get to solstheim,you can also go before that paying 250 gold in the same ship.
Some locations have set items in them. For example, that ebony shield in Labryinthian.
Can you start the Dragonborn questline by doing so?
don't know,didn't ever try,but i think yes
you cant start the quest untill you retrieve the horn of jorgen windcaller, you need the letter from the cultists and they only show up once youve done this.
Actually they show up after your first visit to High Hrothgar, not after retrieving the horn.
I don't think it is possible though. Miraak would reveal that you are dragonborn before you knew it yourself (he says he can sense that you are dragonborn) and that would require a lot of scripting with the main questline.
it isn't possible tried today you need to be atacked by the cultists in Windhelm(well it doesen't have to be windhelm but yesterday was my 17 time in a row lol)