The dogs only alerted to things McCann... And Eddies behaviour changed as soon as he entered 5a.. According to Grime.
So what other explanation is there if Grime and his dogs are so reliable
That's whataboutery. Grime never claimed the alerts proved there was a cadaver in 5A. He said they were indicative that there was a possibility that forensic evidence could be collected from the areas where the dogs alerted. Grime was very clear that it was the evidence that might be collected that was important when working with these dogs. This shows, imo, that he acted professionally. Your only explanation of the alerts has Grime acting with gross professional misconduct for the purpose of feathering his own nest.
Not only does your explanation fly in the face of how Grime conducting himself professionally in this case but it also makes no sense. A series of false alerts do not make his case for the usefulness of EVR dogs and as such would have served him no purpose (assuming, as you seem to, that he wasn't a professional police officer but instead a rogue maverick on the take).