Hi there,
Thanks for sharing your GMAT experience.... as a long-time GMAT teacher and tutor, I'd agree just from a rough assessment that given your scores and practice test scores and self-assessment, it does sound like the nerves have been a challenge. With the 5 tests under your belt I probably wouldn't continue focusing on the exam. With the 640, it would still likely be somewhat of a reach but I also don't know much about you/your profile

.... obviously the GMAT is not everything and schools will be looking at your overall profile and application. 3rd round can be tough, but on the flip side if you are really determined and want to give it a shot, in some ways of looking at it, there's not a ton to lose by giving it a shot, knowing worse case you could re-apply in R1 for example. (And if really needed maybe try a GRE PT if Marshall accepts it and if it didn't work out this time around??). As for the optional essay, you can briefly address why you feel you are well qualified to handle the academics and quant etc of the program or similar. Good luck on your decisions and let us know if we can help further.