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Are you sure it's only because of what happened in April? Usually a country/territory is only gradually won over by the aliens, at least that's been my experience. Usually the ratings you got by a particular government started dropping, and after 3 or more months like that, they changed sides.
Mind you, I'm not sure whether this is always the case, but couldn't that be the reason?
Because from your description it looks like you couldn't do much more -except also cleaning up the small landing sites at the end of April.