2. The blood work comes back, and it contains "unusual substances" (like the babysitting drugs - I forget what they are). See 1 above.
3. Investigate Alexander's medical records -- who, what, when, where, etc. Compare the drugs in his system to the drugs prescribed in his medical records to the drugs the coroner found at the in-laws' home where Alexander's body was discovered.
4. Check Alexander's travel records. How long was he at the in-laws' house?
5. Interview Alexander's other friends and contacts. What did they know about Alexander's recent weeks / months of life?
I bet the wife and the in-laws would break pretty fast under expert police interrogation.