Sorry about your lost colonies. Have you considered sending sample dead bees to the UVM bee lab ? You can google them for details on submissions. There is no charge, except for mailing or delivering the samples.
I suspect your colonies died due to Varroa caused viruses. Formic Pro is not always the best treatment. Did you do any mite counts over the summer ? Especially before and after the Formic ? Formic is also very temperature sensitive and if used at the wrong time (temperature) may cause queen deaths, which will eventually end the colony, especially in the Fall.
Looking at your hives will only show that you have dead bees....it will not determine why. Sending samples to UVM might solve the cause.
Also, we all have many dead bees in the bottom of the hive in Spring. Normal situation.
Good luck and really consider sending samples to UVM.
Peter.