I think it was supposed to be implying that the Ewoks just knocked them down and the finsihed them off violently off screen. It was a PG movie after all. But the fact that it isn't show does strain credulity a bit.

As for the Excecutor crashing into the Death Star, I've alwasy figured the flames are coming from the masssive amount of air that would be released when the Death Star's hull was torn open. The vacuum effect would mean that all of that super-heated air would rush out into space. I'm no physics major, but... is it at all plausible this could result in fire?