I am still a newbie myself and no expert on the AR-15 but did have a similar experience.
As far as having to move the rear sight all the way left, it is probably due to the front sight tower being mounted canted on the barrel. Mine was, but has the 4 set-screw arrangement that allow you to set the rear sight to mechanical zero and then futz with the front tower to get the windage right. From what I understand, if your front tower is the pinned-on type, you just have to be brave enough to plannish to fit (ie, hit it with a big hammer).
I also ran into trouble with the handle mount thing. The first mount I got was a real cheap extruded type (<$20.00). The trouble was, the mount has to seat all the way down in the handle channel and it is a pretty snug interference fit until the handle channel gets spread a little. The cheap mount would screw on, look like it was OK but was not really seated. This made the scope point way above the bore line and could not be adjusted to hit where you wanted it.
I then ordered a more expensive ($50.00) machined mount that I had the same problem until I realized that I was not seating it all the way in like it is suppose to be. Once I got it down flush, everything works fine. In fact, the cheap one probably would have worked OK if I had seated it flush.
Good luck and enjoy your AR-15,
Bill