The majority of the rule clarifications that have come down from Andrew have worked against the players, so I am inclined to say that the diary triggers before the ally. Similarly, a recent post that clarified order of resolution of cards put Horcruxes above player cards.
The only saving grace I can offer you in this situation is that you only lose half the cards in your hand when you are stunned. Cards in the play area do not factor into your discard requirements. There have been a couple of times when I was willing to accept the penalty of being stunned in order to play an ally, so the trick in these situations is to play all of your other cards first and then play the card that will stun you. You still have to drop a control token, but you dodge the discard penalty.
That is correct. The Diary trigger would cause the Hero to be stunned before the Ally's abilities resolve, but at least you can play out your cards in hand so you don't have to suffer the discard effects of being stunned. There have been occasions where it was advantageous to get Stunned on your turn instead of waiting until another players turn, to soften the blow a bit :)
Game Designer - USAopoly