Allow me to paint a scenario to help guide the answering of this question... A Stegadon charges a unit of high elf swordmasters. The impact hits kill 5 of them. High Elves would get to attack next since they're always strike first. However, the 5 impact kills have made it so that they have nobody left in base to base contact with the steggie... so no elves get to attack. Now, does my steggie and crew still get to attack even though nobody is in base contact, but there was at the beginning of the combat?
Yes they still get to attack. When units have done wounds beyond the front rank they can continoue to attack because it's described as they are moving up, pressing the attack. So while the poor General commanding his Swordmasters will wallow is fear and humilitation you'll continoue to unleash terror and death!
Nope. As stated on page 63 of the rulebook, casualties of impact hits don't strike back. Only the unit's champion and an eventual character will strike back. Cheers!