I believe it is an assumption that "he was about to set off with her". For example, as mcdonalds speculates, perhaps that was simply were he confined her while he did other things, or the car was in a building he planned to burn. Who knows?
The McLean's article says,
He later woke her up, assaulted her, put one of her hands in cuffs, dragged her to a replica RCMP Ford Taurus, one of several replica police cars he kept. He shot at her twice to get her to move. He put her in the backseat of the vehicle, along with some containers of gasoline.
That is not necessarily "a few hours".
Lisa McCully was on the beach where she snapped a picture. She sent that picture to her sister shortly after 8PM. So, presumably, at that time, especially if Lisa texted the pic from outside, at the beach, there weren't guns being fired and fires burning vigorously. Later, however, she put her children in the basement of her home for safety and went outside to see what the problem was. My guess is that she was responding to shots fired next door at the Blair house because I don't think she'd have sent her children to the basement for safety if she was responding to smoke and a fire. Greg Blair was shot from outside the home. Jamie inside. There were more than those two shots, as a dog was also wounded. One of the older sons (Tyler) has said there was an AR-15 involved.
Ellison was visiting he father, Richard Ellison who lives in Portapique, but we don't know exactly where. From where it intersects with Orchard Beach Road, for example Portapique Beach Road runs for 1.5 KM, and Orchard Beach Road runs 1.1 KM. I got the impression Ellison might live on Portapique Crescent, which is north of Orchard Beach Road, and would be a bit away from where things were happening. Clinton Ellison told CBC,
But around 10 p.m., they heard a single gunshot. The pair looked outside and noticed a glow in the sky coming from a nearby fire.
Corrie Ellison decided to venture out to find out what was going on, even as his dad pleaded with him to stay. Eventually, he called and told them the fire was really bad and to call the fire department.
His brother and his father waited again. This time, there was no word. Clinton Ellison went out in search for his brother. He walked up the road, flashlight in hand.
"I could see a body laying on the side of the road, as I got closer I could see that it was my brother," he said.
"Eventually, he called" makes me think it took Corrie several minutes to get close enough to see the fire needed the fire department. Clinton and his father also didn't hear the shot(s) that killed Corrie. So, for example, if the Ellison home is on Portapique Crescent (speculation), there are a few hundred feet of trees between there and the nearest fires seen as a "glow in the sky". If he lived further up Portapique Beach Road or Portapique Crescent, they would be 1.5 KM (or more) from the Zahl house.
Although topography probably wouldn't play much of a role in this case, there would obviously be a rise in elevation from the beach to places further from the beach.
Depending on where Richard Ellison's house is and which direction the shooter was firing, it could be not many shots could be heard from there, inside the house, and through the forest. However, it was overcast, which would make the gunfire audible from further.
This video suggests shots with weapons such as those used can be heard up to two miles in ideal conditions:
An RCMP paper evaluating factors affecting how far pistol and rifle shot sound travels:
http://bancroftfishandgame.com/wp-content/uploads/2014/01/Range-Guidlines-sound.pdf