I tried Act 1 as an Oathkeeper to get a feeeling of all the abilities but I don’t want to engage in a full leveling process right now. How do we get the item that allows to skip Normal & Elite ?
You buy them from Asgara at the conclave of the three
in elite you buy to get access to elite and in ultimate to get access to ultimate, similar to crucible difficulties.
The item isn’t in Asgra inventory. Maybe because I already finished act 1 normal ?
No idea, I just went to the conclave of the three with an old lvl 100 char and bought a few for elite unlock and a few for ultimate unlock and put them in my shared stash 
Yep. You’re right. I wonder why you can’t buy it right away.
You don’t get devotion points, no reputation and no resistance buff on low items. That’s weird.