absolutely disrespectful, but I think it's also disrespectful for a dom to use it as punishment. I think regardless of whom it is (whether d/s or not) it's rather rude.
Online is so tricky to begin with- communication can be inherently limited, and when anyone heads out for more than 2-3 days, it seems appropriate to let them know.
Aside from stewing and questioning the relationship, there can be genuine concern over someone's well being. I know I'd get concerned if I didn't hear from someone near and dear to me for almost two weeks.
I like your idea of an essay. It seems appropriate. Anything else, she's liable to enjoy the punishment, particularly if she's been craving attention, hasn't had play for a couple weeks, etc.
Good luck Frozengrapes! I hope it works out ok whatever your punishment ends up being.