They don't have the kind of assembly commonality that the program aimed for initially, but I think it's a stretch to call the three variants "very different" ai

They don't have the kind of assembly commonality that the program aimed for initially, but I think it's a stretch to call the three variants "very different" aircraft. There's a pretty high amount of systems similarity. And pilots don't have to go through completely separate training programs for the three variants, just separate qualifications for things like STOVL or CV operations. If you can aviate in an F-35A, you can aviate in the other two variants. Not sure why you single out the USMC for developing the helmet mounted display, because the HMD is common to all variants and is already a working system.