The more I think about it, foul play seems to be the answer unless they're voluntarily gone missing, but by family and friend's accounts, it doesn't seem likely. Russell and Carol have known each other for many years (40?), they've gone camping together before. It didn't appear to upset the routine of his marriage, his wife either wasn't aware or didn't care. It just doesn't seem viable they'd run off together.
If someone has harmed them, it'd be hard to move 2 dead bodies any kind of distance. If they were escorted by a person/s carrying a rifle/shotgun, it'd be far easier. They could be taken to a mine shaft or were shot and semi-buried under logs and debris.
The fire was extensive, nothing remained. Why would anyone coming across an empty tent burn it to the ground, the owners could return at any time. If the arsonist has harmed Russell and Carol, that does seem more likely. If a charger was the culprit, it wouldn't have happened when Russell was there because why would he then go off to fly his drone?!
If nothing untoward has happened re: a third party, I don't think Carol would've gone searching for the drone with Russell. If he went off to fly the drone and never returned because he'd been injured or suffered a heart attack, she'd be panicking. Would she go seek help from strangers, risking their affair being exposed to Russell's wife, her friend, or would she try to find him, herself? Carol wasn't an experience bush walker, she wasn't 'into' the outdoors, she wasn't used to roughing it, she would have ordinarily stuck to the camp sites and open field areas which were plenty in the area they camped. If she's gone into the thick bush searching for Russell, she could have collapsed from stress and exhaustion and died.
But it doesn't makes sense why neither have been found by searchers or their bodies detected by drones. Why would coincidently, both be well hidden in the bush?