The desing is realisic, the reason why it seems impossible is because we see so many different versions of the VF-1D that we don't know what to believe. Instead of focusing on the Hasegawa type or the Yamato version it is always best to reflect on the original lineart. I know it has so many discrepancies that it is hard to tell what is going on and how things work, but if you really try hard enough and look closely you can see that the answers have always been there. Interpreting the VF-1D is hard but not impossible, it just depends on how you look at it and what you want; perfect battroid, perfect fighter, or perfect GERWALK, the answer and choice is yours.
Eventhough, Yamato and Hasegawa have given us their versions, I must regretfully state they are both very incorrect.
Seeing as how Yamato attempted to work backwards from a two seater to a one seater you can still see how the attempt has failed and would have been best if they had worked the version out the other way around. And in being so it has also changed the original concept of our beloved VF-1A/S/J.
This VF-1D is okay, but far from what it should have been. Nice attempt, but still failed.
Thanks, Yamato.