Actually, the pic is at DTF in the Eu section, and the idea that the Databank has taken an official stance on it is as close to evidence as we have at this moment. While it's not the best source out there, nothing seems to contradict it and it most certainly best fits in with KOTOR 2 and the overall theme of the SW universe.
Those Robes hardly look lightside to me, and really there isn't anything that could explain that mask. He wore it to hide his face that became uglier and uglier because of the Dark Side. At this time most evidence does seem to suggest Light Side, but I really don't consider it a clear and cut case yet. I wouldn't mind either ending really... But the thing is, it just doesn't seem clear enough for me..
Actually, the robes look as ambiguous as possible. Notice you can't see the robes themselves, just the mantle cloak, which has a thick trimming on it I don't recall from Revan's Sith pics. The gauntlets seem to match both sets of robes as well, and the mask doesn't neccessarily have to be his old one. Perhaps Revan prefers masks because he's vain.
The point is either way you try and justify the mask, you're relying on unfounded assumptions, which is sketchy at best.
While it would seem that perhaps he wore a mask to hide his dark sidedness, the answer remains- why would he do so in the first place? Especially when he came back? He was a Sith Lord... he would not need to hide himself from anyone and indeed as you've said facial expressions are key to charisma (Though it's possible he has some kind of charismatic voice that he aids via the Force...).
My explanation theory is that he's vain, or he wears it for tactical reasons. You could say "It's to make the gender and face details open ended" but that isn't a canonical reason that would make sense in-universe.
I don't know why he wore it then, he had it on during his reign as the Dark Lord of the Sith the first time so why not the second time, obviously he had his reasons. Whatever they may have been.
As for the rest I do agree that most evidence suggest he is lightside, but there hasn't been anything "official" to say its that and not the other yet... I still await that before I say he's definitly light side or Dark Side.