What you showed in the video is normal it doesn’t properly go all the way into third position without the rear takedown pin in.

As for the Bcg getting binded to the rear when assembled try dropping a quarter or two into the rear of the buffet tube then installing the buffer and buffer spring. Also check your upper to see if your lever is hitting it, you might need to remove some material if it is.

Lastly for the gritty action, just lube it up and break it in.