First of al OMG! You wrote Mark!

Secondly, this is awesome. The dynamics of Mark and Derek's friendship are so complex, I think, and Mark feels it all so much, buried beneath understatement and deflection, and you've done such a great, in-character job defining that here. (Also very timely, after the exchange at the end of ep 5.14. Mark is still waiting and harming himself in the process.
This most certainly does stand alone. It's a really lovely piece. But now that you've started, I want the longer Mark and Derek fic too!