Nice video! My first time years ago I had a heck of a time getting the carrier back in right. No doubt that gives many first timers trouble and you describe and illustrate it well.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together. You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek