What will you investigate now ? Why are you including such a stupid theories of death. You'll do nothing if you repeat the investigation probably. The most important procedure and the first thing leading to good result in all of the crimes happening is the forensic examination on the real crime scene, the real time, done by experienced, prepared, autonomous (in my opinion in Soviet Russia every information before showing to public, must be changed due to command from the russian administration) forensic team of specialists in their spheres, like medical forensic pathologists, etc.
The investigation which was done 60 years before was inappropriate and done improperly. 90% of these theories could be excluded after the investigation on the crime scene, after the analyzing the results!
What about methyl alcohol poisoning? Its very easy to prove someone has died of it. Do you have information about their autopsies? Were autopsies on their bodies held ? Where are the results from them ? Are they real, or exaggerated?
Methanol poisoning can be proven by biochemical and histopathological analysis of the tissues of deaths. How can you prove methanol poisoning after 60 years ? How can you answer this question now? Tellings from people, stories from diaries, lies from people, you can not gain real information, the good forensic evidences are buried with bodies and disappeared forever! In the bones you can not find methanol. This is the reason why they tell 'In reality, those rare few cases with good forensic evidence are the ones that make it to court'.