As to question 2, that's about the right sequence. I remember talking with a S&W distributor around 2014, who at that time said they were unsure if they would ever get another shipment of new 3d Gens because the Houlton pistol assembly operation was downsized and the equipment was being moved back to Springfield.

I would guess that they never fully restarted the assembly line and the later Springfield pistols were simply finishing the remaining works in progress when they moved, hence the different style serial number marking.