AT&T can throttle individual lines after 22gb of usage on that line. So if you have 4 lines on the plan and one line goes over 22gb in a billing period, only that line is affected, for the remainder of the same billing period, and throttling is possible in high traffic areas.
We've had this plan for about 3 months now, and have 5 lines on our plan. Three times in the 3 months has any line gone over 22gb, and none have actually been throttled to any perceptible level.
You do not have to combine AT&T and DirecTV billing, but you do get a discount if you do. If for any reason you cancel DirecTV, you must choose another plan from the available offerings. There's no guaranteed "go back" to where you were before selecting this plan.
Hotspot is not possible.