I think you should use the optional essay but be careful. Don't make it sound like an excuse. Don't try to explain away your 2.7. Make it proactive and compelling. Unless, of course, you had a REALLY good reason you got a 2.7 but generally most applicants don't.
However, you WERE proactive so you should definitely highlight that. There is a clear story here that shows your dedication to your goals. Additionally, I'm sure they could piece it together but why take that risk? Spell it out in a clear and concise manner. You don't have many words to play with so make them count.
And good for you for taking those steps to remedy the GPA!
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