Bits of progress everywhere.
On the flat coat going chunky issue, not sure of the type of paint you used, but I have found with both Gunze Acrylics and even SMS Laquers, if the flat gets the white fleck business, a light overspray with neat, undiluted Laquer Thinner will most times help to dissolve the chunks, which I believe are the flat pigment that has lumped together and not mixed properly and/or dried before hitting the surface in more hostile painting conditions, which I agree High Humidity is one.