Here are both versions for ease of direct comparison. My guess is that either Cicero alone is correct, or Diogenes is also correct and multiple letters were sent out.
Gedney noted (as you did) that the letter is addressed to υμιν "you all" (plural 2nd person), so the letter could have been addressed to the members of the school or the "governing board" if you will, which could have included both Hermarchus and Idomeneus. Cicero is closer in time to Epicurus than Diogenes Laertius, so maybe giving him the upper hand in authenticity is warranted??