Hi. None of the options you suspect above are true. And what you were told is true, generally seat selection at T-48.
Just because you didn't get to select a seat does not mean you were not assigned a seat, generally they put me in last row, middle.
Ah, if it was only that simple. Seat selection can also be affected by fare class and elite status and codeshare and... We might need a FAQ on this... anyone?