About the dog: Lucky is described as an overweight, 100lb Akita mix. Overweight or not, the Akita was bred for hunting large mammals, including bear and boar, and as a personal protection dog.
Since Lucky is described as devoted to his owner, it's a fair bet that anyone attacking Patrick would have had Lucky to deal with first.
1. Lucky was shot and killed while defending Patrick - so why has the dog's body not been found during the searches of the area even if Patrick was taken away and disposed of elsewhere? There seem to have been no reports of blood being found at the scene, which you would expect if either Lucky was shot or he attacked Patrick's attacker.
2. Lucky was shot and injured while defending Patrick, but was not sufficiently badly injured to kill him immediately or quickly - this might explain the passing trucker's report that he saw a dog matching Lucky's description 3 days later just north of exit 205. But why was Lucky not found during searches of the area after Patrick's car was found?
3. Lucky was not shot at all, which suggests that for some reason he did not, or was not able to, defend Patrick from attack, and if not why not?
I can't help thinking that the most likely scenario is that Patrick wandered away from his car and had a sudden health emergency, and that Lucky, unable to revive him, returned to the area of the interstate in an attempt to attract attention. But this would mean that Patrick was somewhere outside of the search area and doesn't really explain what happened to Lucky in the end. If Lucky was nearby and trying to attract attention, he would surely have gravitated to the activity at the ramp while the search was going on.
I dunno, this doesn't really add up.