Does the +1 (or +2 for steep) modifier for defenders who are uphill from attackers only apply to the 1st round of assault? I assume so, and the chart doesn't say.
Yes, the modifier applies only on the first turn of the assault.
The attackers and defenders in the same elevation in subsequent turns.
Concur.... the advantage to the defender is the disorganization of the attackers as they try to negotiate the slope, while being fired upon (and presumably returning fire). Once they have crested the slope, and are on level ground with the defenders, then it just becomes a bit of a slobber-knocker CQB situation.