1)The advantage is extra damage. It's very noticeable if you have the numbers showing as you hit from behind.
2) I don't know the arc. They're very unclear about this. I'll be diagonal to a large bear and feel like I don't know if I'm getting anything because bears have enough armor that the whole party's damage goes down from what it normally is (so no normal damage amount to compare with), and I can't really tell a huge difference between flank and non-flank animations, assuming there is one of course. DA: Origins gave you a better explanation and visual confirmation via the animation that you were flanking, whereas if I am flanking in this game I really cannot tell by anything other than doing a lot more damage randomly.
3) I agree. There's a skill called "Flank Strike" or "Flanking Strike" or something along those lines which, if you're standing in front (not behind) of a person then it will make you attack them and end with you facing backwards. That is not an ideal solution of course. In the tactical menu you just have to position yourself behind, or navigate there by moving. I don't think it's a particularly elegant system by any stretch, and it really makes me think that rogue is the victim of the new engine. Everything about being a rogue in this game seems like it has a disadvantage, but that's a post for another time.
The most important point is that if you trigger skills in the tactical menu for rogues, they miss constantly, and they never go for a flank themselves in tactical mode, which is a bit ridiculous because the control is clunky to begin with in both modes as far as navigating rogues is concerned.