Could my character actually use the whole "bury a dead body down six feet deeper than you would, and bury a dead animal at the normal burial depth" thing, or since that's slightly common knowledge and as you've mentioned before, it's hard to lose the trained dogs that sniff these things out, would the police dig deeper because they trust the animal?
I mean for a book, by all means go for it. Make your characters dig a 12-foot deep hole by hand. There is probably some way you could imagine for them to not be discovered. Please do note that is is really difficult to dig a hole though, you got to make sure that it won’t collapse in on itself, and that your character has a way out (like even a regular 6-foot hole is difficult to climb out of).
Most bodies are discovered in shallow graves, because it is HARD AS HELL to dig a long deep, body sized hole, and not be noticed by someone. Sniffer dogs are very well trained, and are trained to ignore dead animal smells, and are looking for dead human smells. These dogs can differentiate. I don’t know how. But they have AWESOME noses.
What you may not know is that dogs usually don’t find these graves. They are found because the murdered confesses, someone decides to put a well in on their farm, an aerial sweep detects the anomaly in the dirt, or because a walking sweep finds disturbed earth and they are looking for a body.
Watson: Here is an article on human remains detection (HRD) dogs. In this article, dogs were able to sniff out the location of where a body was buried after it has been moved. It goes in-depth in regards to how sensitive a trained dog is, and the kind of signals that are used to communicate with its handlers. There isn’t any mention of faking with another animal, but they probably will be looking for specifically human death smell.
Now for some speculation: For a novel, there are hundreds of possibilities you can think up to use the whole “bury a dead body extra deep and put a dead animal on top so no one can find the body”. You could play with remote locations so you character is not discovered burying the body, and so if the body is ever found there is no connection. You could have small town cops find the site, reach a dead family pet and stop because they don’t want to disrupt the poor pups remains. The possibilities end where your creative deviance does. For real life though (one don’t murder people) this is not really a plausible way to hide a body.