NO. It's not at all a good idea.

HBS has provided you only 800 words to present your self and you don't want to use all of them by discussing just one accomplishment, that too, with flaws.Frankly speaking,I don't understand how one story can answer both the essay prompts ( 1."Tell us about something you did well" and 2."Tell us about something you wish you had done better"?)

Again, I strongly recommend that you select two different stories for both essays

Dude, just remember the most important thing is what you say, not necessarily what you choose to talk about. The topics are very broad to allow you flexibility in your response. This doesnt mean when they ask "something" it has to be "one thing" or that something you did well cant also be something you wish you had done better. The consultants are right in their guidance without knowing specific details, but talk about what you really want to talk about. If you are honest and sincere you will get in where you deserve to get in.