An omission: I did not complete my Punnet square, but you cover it exhaustively. That #2 is a possibility, something far beyond my ability to flesh out. A chance discovery unable to be explained by the discoverer. It reminds me of a Harry Turtledove short story, "The Road Not Taken," about first contact where we accidentally skip over a common technological development. PDF: http://www.eyeofmidas.com/scifi/Turtledove_RoadNotTaken.pdf

Also re: #4 - for what it's worth (and a great entertaining read), Harry Turtledove covers that one too in his "In the Balance" book, where alien invaders show up during WWII. Highly recommend it.