The decomposition is not the main problem. The main problem, is that if poisoning is suspected, they might not find anything after 7 weeks of being buried, especially considering body was embalmed, which can interfere with toxicology. For toxicology, blood and other fluids need to be collected as soon as possible, even better if the person is still alive. That's how they got the guy who poisoned his wife with cyanide, she had blood collected while still alive. If that hasn't happened, chances are that guy would be walking free now.