Since there is virtually no location context for that picture of the door, doorknob & boot print, isn't it possible that the print is on the interior of the door? AFAIK, no one has reported that anyone witnessed anyone rushing away and/or driving away from the house. But, we must assume that once the gas was poured, the matches lit and then dropped at the top of the stairs, the perpetrator would be in somewhat of a hurry to get the heck out of this crime scene. The side or a back door would provide more privacy....as opposed to the garage or the front doors. And, from the back of the house one could escape down the walkway between the S's driveway and the neighbor's driveway, correct?
Now, about those matches left at the top of the stairs: It is very hard to strike a match with gloves on. Since the matches were found on or at the top of the stairs, maybe there will be fingerprints on the matchbooks or box.