in the very first episode, when he first appears, Allelujah is about to launch from the ship.
He says: "It's a real battle. Hallelujah. Tired of waiting?" Then he sighs and says, "I'm depressed." and puts on his helmet.
Now it's possible he was talking to his Gundam. But it is certainly concievable that what was translated as "It's a real battle. Hallelujah" could also have been "It's a real battle, Hallelujah".
In fact, I'm much more inclined to think he used the word 'Hallelujah' to refer to himself than as an expression of joy/rejoicing, particularly because he immediately declares himself depressed (about the battle). He also voices concern about the Gundam's missions and their overwhelming force in almost every episode, so it's almost for sure that he uses the word to refer to himself in the third-person.
That and the fact that he asks himself a question... in his very first appearance in all of the show...
Plus, in episode 5, when he's on the rail train up to the space station, the attendant comes in and says, "Traveling by yourself?" And how does he reply? With: "I enjoy looking at the Earth from above."
At first glance I thought this was a shrewd way of him concealing potential traveling mates, potential information which could be used if he's ever ID'd as a Gundam pilot while others are not. But, considering what happens later, and the fact that he's clearly not with any of the others at that point, it is another strong indication that his split-personality is something that doesn't just surface out of nowhere from the Newtype experience.
So, yeah, i think that answers the question on whether or not he shows signs of AMPD before this event.
edit - also of note: After Allelujah switches over to his 'right side personality' he asks "Who the hell is this?" They cut to the HRL Soldier and she asks, "Who is this?" To which Hallelujah screams "Bastard, I'm going to kill you!" The girl, as if hearing this (and she certainly does at least sense this threatening intent) looks very afraid and starts firing at the station's port, where Allelujah has just gotten off of his flight.
Also, you can see the difference in his personalities largely in his reaction to seeing the broken-off gravity block floating in space:
"An accident?" He laughs evilly. "My sincerest condolences" (obviously doesn't mean it).
Also, you'll notice Mendoi / Conclave sometimes have it 'Allelujah' and other times "Hallelujah". From the fact that the boy in his flashback refers to him as 'Allelujah' and that the 'good voice' addresses the 'evil right side' as 'Hallelujah', I think we can go off of these two names for each personality.
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