Another possibility I haven't seen many people speak of is that this might not be a foreign adversary, but a test by our government itself. Our government has a long history of testing experimental weapons on its own personnel, such as the MK Ultra Project and Operation Top Hat. I wouldn't put it past one of our military or intelligence agencies to do this. https://en.wikipedia.org/wiki/Unethical_human_experimentatio…
I've said for years that the Havana Syndrome is just the latest of Syndromes related to workplace injuries. (Previous ones were caused by Agent Orangein Vietnam, Gulf Syndrome from the Iraq War of 1991, Burn-Pit lung-damage from Iraq 2003, Afghanistan, etc, etc, etc.)
Something all those spooks were doing in their diplomatic posts is the cause of Havana Syndrome. But the Government will hide any evidence they have, so that there won't be millions or even billions of dollars in Workplace Injury Compensation claims.