Actually- just something of note here -in the story of Beowulf he brings along eleven warriors, of whom ten then flee when the dragon emerges and puts up a fight, leaving Wiglaf and Beowulf to fight it themselves. In that scenario, a lone, relict Gelupterus could still be the dragon. It is also interesting how much of an emphasis these early stories put on mentioning venom along with the fire- which adds credence to your method of explaining that part of the legend.