The greatest thing about the comeback is the ability to sight adjust on the fly...
If it's press coverage, I'll wait til the receiver breaks jam and lead him based on his position after the break. If he breaks inside, I'll fire a bullet while the defender is still trailing. If he breaks outside, I'll lob the ball toward the sideline and allow him to run toward it.
If the CB is in a Deep Zone, I'll wait until the receiver turns then throw a line drive to the comeback point.
Against normal and loose man coverage, I'll throw the ball just before the WR makes his break. The DB will usually take two more steps upfield before turning leaving the WR alone to make an unmolested catch.
The only coverage I avoid is the dreaded buzz zone (purple) because there is usually help that limits the throwing lane. If it's a single defender though, I'll take a chance and user catch it 1-on-1.
As far as the rush, I live in I-Form. My backs take the outside rushers and I let the big guys take the inside rushers. When I maintain my pocket presence I can usually get time to get the pass away (assuming I'm setting up the comeback).
I like running comebacks to the wide side of the field... The more room there is to defend, the more there is to throw. I set this up by running often to the short side when I've stacked receivers on the wide side. after a few decent gains, the comeback/post combination from Twin WR works WONDERS.
Later