Yeah, the film pretty much does state the PredAlien took a siesta until they escaped the sewers.
Also the homeless lady probably would have been impregnated, but only with one Alien. Buddy and Sam, two bums and the homeless lady make five Aliens. Wolf kills two, three escape into the street with the PredAlien.
So Chet's first batch of pregnant bursters is, again, the waitress, who doesn't pop until shit is already going down.
And the endless cars was because of the power plant, not the Aliens.